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HUMAN  PHYSIOLOGY 


Digitized  by  tine  Internet  Arciiive 

in  2010  witii  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/humanphysiologyeOOpear 


HUMAN  PHYSIOLOGY 

ESPECIALLY  ADAPTED  FOR 

DENTAL   STUDENTS 


BY 


R.  G.  PEARCE,  B.A.,  M.D., 

Assistant  Professor  of  Physiology,  University  of  Illinois 


AND 


J.  J.  R.  MACLEOD,  M.B.,  D.P.H., 

Professor  of  Physiology.  Western  Reserve  University 


SECOND  REVISED  EDITION 


FIFTY-NINE  ILLUSTRATIONS,  INCLUDING  TEN  COLOR  PLATES 


ST.  LOUIS 

C.  V.  MOSBY  COMPANY 

1916 


1  ^  ii 


Copyright,  1916,  by  the  C.  V.  Mosby  Company 


Press  of 

The  C.  V.  Mosby  Company 

St.  Louis 


PREFACE  TO  SECOND  EDITION. 

The  gratifying  reception  which  the  first  edition  of  this  book 
has  met  with  and  the  kindly  criticism  of  our  colleagues  have 
encouraged  us  to  revise  it  carefully,  with  the  particular  object 
of  presenting  in  simpler  form  some  of  the  more  difficult,  and  yet 
essential  principles  upon  which  the  modern  science  of  physiol- 
og;^^  depends.  This  has  been  done  without  changing  the  paging 
of  the  book,  and  therefore  without  in  anywise  altering  its  gen- 
eral nature. 

Although  the  book,  as  stated  in  the  preface  to  the  first  edi- 
tion, has  been  specially  written  to  meet  the  requirements  of  the 
student  of  dentistry,  the  principle  has  been  constantly  kept  in 
mind  that  the  physiology  which  he  should  learn  is  essentially 
the  same  as  that  required  by  the  student  of  medicine.  To  em- 
phasize this  point  the  title  of  the  book  has  been  somewhat  altered 
in  form. 

The  outlines  of  experiments  found  in  the  appendix  are  ar- 
ranged to  occupy  about  sixty  hours  of  laboratory  work.  While 
the  authors  realize  the  impossibility  of  formulating  experiments 
suitable  to  all  laboratories,  nevertheless  they  hope  that  these 
will  be  of  some  service. 

The  authors  are  deeply  indebted  to  Miss  Achsa  Parker  and 
Dr.  E.  P.  Carter  for  pointing  out  many  of  the  errors  that  ap- 
peared in  the  first  edition. 

R.  G.  Pearce. 

J. 'J.    R.    MACLEOD. 


PREFACE  TO  FIRST  EDITION. 

A  knowledge  of  the  fundamentals  of  human  physiologj'  is 
essential  in  the  training  of  the  dental  student,  because  physiology 
constitutes,  along  with  anatomy,  the  basic  science  upon  which 
all  medical  and  surgical  knowledge  is  founded ;  and  dentistry  is 
a  highly  specialized  department  of  surgical  practice.  To  oper- 
ate on  the  teeth  without  knowing  something  about  the  physi- 
ology of  the  body  as  a  whole,  would  reduce  the  dentist  to  the 
level  of  a  craftsman  who,  although  perhaps  very  highly  skilled 
in  his  technical  work,  was  yet  quite  ignorant  of  the  nature  of 
the  machine  upon  a  part  of  which  his  work  had  to  be  done. 

But  there  are  also  practical  reasons  why  the  dentist  should 
be  familiar  with  physiology,  for  good  health,  and  not  good  looks 
alone,  depends  very  largely  on  sound  teeth.  The  neglect  of  this 
fact  may  cause  disturbances  in  bodily  functions  to  which,  at 
first  sight,  the  teeth  may  apparently  bear  very  little  relation- 
ship ;  thus,  extreme  emaciation,  with  its  consequent  lowering  of 
the  normal  resistance  of  the  body  towards  disease  and  infection, 
is  well-known  to  be  frequently  due  to  no  other  cause  than  some 
abnormal  or  pathological  condition  affecting  the  teeth ;  and,  on 
the  other  hand,  this  very  condition  itself  may  become  intract- 
able to  the  most  skilled  dental  treatment  and  hygiene,  if  meas- 
ures are  not  taken  at  the  same  time  to  improve  the  general  health. 
Although  it  is"  obviously  beyond  the  province  of  the  dentist  to 
undertake  the  treatment  of  these  general  conditions,  yet  it  is 
most  important  that  he  should  be  sufficiently  familiar  with  the 
normal  functioning  of  the  human  body  to  be  able  to  recognize 
what  is  really  at  fault.  A  knowledge  of  the  laws  of  nutrition 
and  dietetics  must  therefore  form  a  most  important  part  of  every 
course  in  dentistry,  and  these  have  received  particular  attention 
in  this  book. 

The  physiology  of  the  digestive  system,  of  the  circulation  of 
,  the  blood  and  of  the  nervous  system  is  scarcely  less  important. 


PREFACE.  V 

The  pain  and  shock  produced  by  a  dental  operation  may  cause 
considerable  disturbance  in  the  action  of  the  heart  or  in  the  dis- 
tribution of  blood  in  the  body,  and  this  disturbance,  especially 
in  cases  in  which  the  heart  and  the  blood  vessels  are  diseased,  may 
become  so  pronounced  as  to  render  a  certain  amount  of  medical 
skill  necessary.  Or  if,  to  avoid  such  pain,  it  be  deemed  advisable 
to  administer  anesthesia,  then  must  the  dentist  be  constantly 
on  his  guard  that  no  more  than  the  proper  amount  of  anesthetic 
is  given,  which  he  can  do  intelligently  only  by  observing  the 
condition  of  the  nervous  and  circulatory  systems. 

Besides  knowing  something,  about  the  physiology  of  the  body 
as  a  whole,  the  dentist  must  be  particularly  familiar  with  the 
local  physiology  of  tlie  mouth,  such  as  the  finely  coordinated 
nervous  mechanisms  involved  in  the  acts  of  mastication  and 
swallowing  and  the  secretion  of  saliva.  He  must  understand 
the  nature  of  the  sensations  of  the  teeth  and  buccal  mucosa,  and 
be  on  the  lookout  for  any  lesions  of  the  cranial  nerves  that  sup- 
ply the  muscles  and  other  tissues  adjacent  to  the  mouth  cavity. 

The  chemistry  of  the  saliva  has  demanded  special  attention 
because  of  the  very  interesting  scientific  investigations  which 
are  being  prosecuted  regarding  the  nature  of  the  undoubted 
relationship  that  exists  between  changes  in  the  saliva  and  the  in- 
cidence of  dental  caries.  To  adequately  describe  the  present 
status  of  this  work  we  have  found  it  necessary  to  devote  some 
space  (in  the  second  chapter)  to  a  review  of  the  main  physico- 
chemical  principles  which  may  regulate  the  reaction  and  neu- 
tralizing power  of  saliva.  , 

Whenever  the  occasion  presented  itself  to  do  so,  we  have  given 
a  brief  description  of  the  general  nature  of  the  diseases  in  which 
dental  involvement  is  possible. 

A  few  simple,  but  very  instructive,  laboratory  demonstra- 
tions are  described  in  an  appendix  at  the  close  of  the  book.  "We 
have* found  that  such  demonstrations  furnish  an  invaluable  aid 
in  the  leaching  of  the  subject. 

To  facilitate  a  ch'ar  understanding  of  the  subject,  diagrams 
have  been  used   whenever  necessary,  and  many  of  these  have 


VI  PEEPACE. 

been  specially  drawn  for  the  work.  To  Prof.  T.  Wingate  Todd 
and  Mr.  P.  M.  Spurney,  the  authors  are  deeply  indebted  for 
the  valuable  assistance  which  they  gave  in  the  preparation  of 
these. 

R.  G.  Pearce. 

J.  J.  R.  MACLEOD. 


CONTENTS. 

Chapter  I. 

THE  CHEMICAL  BASIS  OF  THE  CELL. 

Page 
The  Scope  of  Physiology — The  Physico-chemical  Basis  of  Life — 
The   Chemical   Basis   of   Animal    Tissues — Water — Proteins — 
Lipoids — Carbohydrates    17 

Chapter  II. 

THE  INFLUENCE  OF  PHYSICO-CHEMICAL  LAWS  ON 
PHYSIOLOGICAL  PROCESSES:     ENZYMES. 

Properties  of  Crystalloids — Osmotic  Phenomena  in  Cells — Reac- 
tion of  Body  Fluids — Colloids — General  Nature  of  Enzymes 
or   Ferments    ' 26 

Chapter  III. 

DIGESTION:    NECESSITY  AND  GENERAL  NATURE. 

Digestion  in  the  Mouth — The  Salivary  Glands — The  Nerve  Supply 
of  the  Salivary  Glands^ — The  Reflex  Nerve  Control  of  the  Sali- 
vary Secretion — The  Normal  Stimulus  for  Salivary  Secretion 
(Direct  and  Psychological) — General  Functions  of  Saliva 37 

Chapter  IV. 

DIGESTION:      THE    CHEMISTRY    OF    SALIVA    AND    THE 
RELATIONSHIP  OF  SALIVA  TO  DENTAL  CARIES. 

Organic  and  Inorganic  Constituents — The  Reaction  of  Saliva — The 
Method  of  Measurement  of  Neutralizing  Power  of  Saliva— 
The  Deposition  of  Tartar  and  Calculi 46 

Chapter  V. 

DIGESTION. 

Mastication — Deglutition   or    Swallowing — Vomiting 53 


Viii  CONTENTS, 

Chapter  VI. 

DIGESTION:     IN  THE  STOMACH.  Page 

Mechanism  of  Secretion  of  Gastric  Juice — The  Active  Constituents 
of  Gastric  Juice — The  Movements  of  the  Stomach — The  Open- 
ing of  the  Pyloric  Sphincter — Rate  of  Discharge  of  Food  from 
the  Stomach 60 

Chapter  VII.   ' 

DIGESTION:      IN  THE  INTESTINE.     ^ 

Secretion  of  Bile  and  Pancreatic  Juice — Functions  and  Composi- 
tion of  Pancreatic  Juice  and  Bile — Chemical  Changes  Produced 
by  Intestinal  Digestion — Bacterial  Digestion  in  the  Intestine — 
Products  of  Bacterial  Digestion — Protection  of  Mucous  Mem- 
brane of  Intestine  Against  Autodigestion — Movements  of  the 
Intestines — The  Absorption  of  Food — Resume  of  Actions  of 
Digestive  Enzymes  71 

Chapter  VIII. 

METABOLISM:   ENERGY  BALANCE. 

Introductory — General  and  Special  Metabolism — Energy  Balance- 
Caloric  Value  of  Foods — Basal  Heat  Production — Influence  of 
Food,  Muscular  Work,  Atmosphere,  and  Size  of  Body 83 

Chapter  IX. 

METABOLISM:   THE  MATERIAL  BALANCE  OF  THE  BODY. 

Starvation-Nitrogen  Balance — Protein  Sparers — The  Irreducible 
Protein  Minimum — Varying  Nutritive  Values  of  Different 
Proteins 91 

,  Chapter  X. 

THE  SCIENCE  OF  DIETETICS. 

The  Proper  Amount  of  Nitrogen — Chittenden's  Experiments — The 
Most  Suitable  Diet  for  Efficiency — Chemical  Composition  of 
the  Common  Foodstuffs  99 


CONTENTS.  IX 

Chapter  XI. 

SPECIAL   METABOLISM.  Page 

Special  Metabolism  of  Proteins — Urea — Ammonia — Creatinin — 
Purin  Bodies — Relative  Importance  of  Proteins,  Fats  and 
Carbohydrates  in  Metabolism 108 

Chapter  XII. 

SPECIAL  METABOLISM. 

Metabolism    of   Fats — Metabolism   of   Carbohydrates — Metabolism 

of  Inorganic  Salts — Vitamines  115 

Chapter  XIII. 

THE  DUCTLESS  GLANDS. 

Introduction — Thyroid  and  Parathyroid  Glands — Adrenal  Glands^ 

Pituitary  Gland — Spleen — Thymus  Gland 124 

Chapter  XIV. 

ANIMAL  HEAT  AND  FEVER. 

Animal  Heat — Normal  Temperature — Factors  Concerned  in  Main- 
taining the  Body  Temperature — Regulation  of  Body  Tempera- 
ture— Fever  134 

Chapter  XV. 

THE  BLOOD. 

Introduction — Physical  Properties — The  Corpuscles — Erythrocytes 
— Haemoglobin — Enumeration  of  Blood  Cells — The  Origin  of 
the  Erythrocytes — The  White  Cells — Leucocytes — Lympho- 
cytes— Functions  of  the  White  Cells — The  Blood  Platelets — 
The  Blood  Plasma 140 

Chapter  XVI. 

THE  BLOOD. 

The  Defensive  Mechanism  of  the  Blood — Coagulation  of  the  Blood 
— Antibodies  in  the  Blood — The  Process  of  Inflammation — 
Toxins — Antitoxins — Ehrlich's  Side  Chain  Theory— Anaphy- 
laxis— Phagocytosis — Opsonins  147 


X  CONTENTS. 

Chapter  XVII. 

THE  LYMPH.  Page 

Lymph  Formation— Lymphagogues— Lymph  Reabsorption— The 
Movement  of  Lymph 155 

Chaptek  XVIII. 

THE  CIRCULATION. 

Introduction — The  Heart — Anatomical  Considerations — Physiologi- 
cal Properties  of  Heart  Muscle — Character  of  Cardiac  Con- 
traction— The  Sequence  of  the  Heart  Beat — The  Action  of  , 
Inorganic  Salts  on  the  Heart — The  Vascular  Mechanism  of  the 
Heart — Definition  of  Terms — Events  of  the  Cardiac  Cycle — 
The  Heart  Sounds — Diseases  of  the  Cardiac  Valves 159 

Chapter  XIX. 

THE  CIRCULATION. 

The  Blood  Flow  Through  the  Vessels — The  Part  the  Heart  Plays — 
The  Part  the  Vessels  Play — Arterial  Blood  Pressure — Factors 
That  Maintain  the  Blood  Pressure — Velocity  of  Blood  Flow — 
The  Return  of  the  Blood  to  the  Heart — Circulation  Time — 
The  Effect  of  the  Circulation  of  the  £lood  Itself— The  Pulsa- 
tile Acceleration  of  the  Blood  Flow — The  Pulse — The  Circula- 
tion in  the  Lungs 171 

Chapter  XX. 

THE  CIRCULATION. 

The  Influence  of  the  Nervous  System  on  the  Circulation  of  the 
Blood — The  Nervous  Control  of  the  Heart — The  Cardiac 
Nerves — Accelerator  Nerves — Inhibitory  Nerves— Interrelation 
of  Inhibitory  and  Accelerator  Nerves — The  Cardiac  Center — 
The  Cardiac  Depressor  Nerves — The  Nervous  Control  of  the 
Blood  Vessels — Vasomotor  Nerves — ^Vasoconstrictor  Nerves — 
Vasodilator  Nerves — Vasomotor  Reflexes — The  Effect  of  Grav- 
ity on  the  Circulation — Haemorrhage — Chemical  Control  of 
Circulation — Asphyxia — Nitrous  Oxide — Cocain  184 


CONTENTS.  Xi 

Chapter  XXI. 
THE  RESPIRATION.  Page 

Introduction — The  Internal  Respiration — Oxidation  in  the  Tissues 
— Relation  of  Oxidative  Process  to  Muscular  Activity — Physi- 
cal Laws  Governing  Solution  of  Gases — Hsemoglobin — Rela- 
tion of  Oxygen  to  Haemoglobin — The  Mechanism  of  the  Res- 
piratory Exchange — The  Effect  of  Carbon  Dioxide  on  Oxy- 
hsBmoglobin — The  Exchange  of  Carbon  Dioxide 197 

Chapter  XXII, 

THE  RESPIRATION. 

The  External  Respiration — Structure  of  the  Lungs — The  Mechan- 
ism of  the  Respiratory  Movements — The  Part  the  Diaphragm 
Plays — The  Part  the  Thorax  Plays — The  Movements  of  the 
Lungs — Respiratory  Sounds — Effects  of  Respiration  on  the 
Circulation — Artificial  Respiration — Volumes  of  Air  Respired 
— Mechanism  of  Gaseous  Exchange  in  Lungs 207 

Chapter  XXIII. 

THE  respiration: 

The  Nervous  Control  of  the  Respiration — Reflex  Respiratory  Move- 
ments— Chemical  Control  of  the  Respiration — The  Effect  of 
Changes  in  the  Respired  Air  on  the  Respiration — Mountain 
Sickness — Ventilation — The  Voice — Mechanism  of  the  Voice — 
Speech 219 

Chapter  XXIV. 
THE  FLUID  EXCRETIONS. 

The  Excretion  of  Urine — Composition  of  Urine — Organic  Constitu- 
ents— Urea — Ammonia — Uric  Acid — Creatinin — Inorganic  Con- 
stituents— Abnormal  Constituents — The  Organs  of  Excretion 
— The  Blood  Supply  of  the  Kidney — Nature  of  Urine  Excretion 
— Micturition — The  Secretions  of  the  Skin — The  Sweat  Glands 
— The  Sebaceous  Glands — The  Mammary  Glands 229 


Xii  CONTENTS. 

Chapter  XXV. 
THE  NERVOUS   SYSTEM.  Page 

General  Nature  and  Structure  of  the  Nervous  System  in  Different 
Groups  of  Animals — Fundamental  Elements  of  the  Reflex  Arc 
— Integration  of  the  Nervous  System 239 

Chapter  XXVI. 

THE   NERVOUS   SYSTEM. 

Reflex  Action— The  Nerve  Structures  Involved  in  the  Reflexes  of 
the  Higher  Animals — The  Receptors  of  Pain,  Touch,  Tempera- 
ture— Local  Anesthesia  and  Analgesia — The  Afferent  Fiber — 
Choice  of  Paths  on  Entering  Spinal  Cord — The  Nerve  Center 
— The  Efferent  Neurone — Types  of  Reflexes — Spinal  Shock — 
The  Essential  Characteristics  of  Reflex  Action — Muscular 
Tone  and  Reciprocal  Action  of  Muscles — Symptoms  Due  to 
Lesions  Affecting  the  Reflexes 244 

Chapter  XXVII. 

THE   NERVOUS   SYSTEM. 

The  Brain  Stem — The  General  Course  and  Functions  of  the  Cranial 
Nerves,  Particularly  of  the  Fifth  and  Seventh — Relationship 
of  the  Fifth  Nerve  to  the  Teeth  and  to  Neuralgia — Referred 
Pain  Through  this  Nerve — Sensitiveness  of  the  Tooth — Tri- 
facial Neuralgia — Relationship  of  the  Seventh  Nerve  to  Bell's 
Paralysis 256 

Chapter  XXVIII. 

THE  NERVOUS  SYSTEM:      THE  BRAIN. 

Influence  of  the  Brain  on  the  Reflex  Functions  of  the  Spinal  Cord 
— ^Functions  of  the  Cerebrum — Cerebral  Localization — Experi- 
mental and  Clinical  Observations — The  Sensory  Centers — The 
Mental  Process — Aphasia — The  Cerebellum — Relationship  to 
Body  Equilibrium — The  Semicircular  Canals — The  Sympa- 
thetic Nervous  System— General  Characteristics — The_  Course 
of  Some  of  the  Most  Important  Pathways 267 


CONTENTS.  XUl 

Chapter  XXIX. 

THE  SPECIAL  SENSES:     VISION.  Page 

Optical  Apparatus  of  the  Eye — Formation  of  Retinal  Image — 
Changes  in  the  Eye  During  Accommodation  from  Near  Vision 
— The  Function  of  the  Pupil — Imperfections  in  the  Optical 
System  of  the  Eye — ^Long  and  Short-Sightedness — Astigma- 
tism, etc. — The  Sensory  Apparatus  of  the  Eye — The  Functions 
of  the  Retina — Blind  Spot — Fovea  Centralis — The  Movements 
of  the  Eyeballs — Diplopia — Judgments  of  Vision — Color  Vision 
—Color  Blindness  279 

Chapter  XXX, 

THE  SPECIAL.  SENSES. 

Hearing — The  Cochlea — How  Sound  Waves  are  Transmitted  to 
this  by  Tympanic  Membrane  and  Auditory  Ossicles^Causes 
of  Deafness — Taste — Nature  of  Receptors  for  Taste — The 
Location  of  the  Four  Fundamental  Taste  Sensations — Rela- 
tionship Between  Chemical  Structure  and  Taste — ^Association 
Between  Taste,  Common  Sensation  of  Touch,  and  Smell — 
Action  of  Certain  Drugs  on  Taste — Smell — Nature  of  the  Re- 
ceptors of  Smell  (the  Olfactory  Epithelium)  —  Nature  of 
Stimulus 291 

Chapter  XXXI. 

THE  MUSCULAR  SYSTEM. 

The  General  Properties  of  Muscular  Tissues  —  Contractility  — 
Irritability — The  Simple  Muscular  Contraction — Tetanic  Con- 
traction—Effect of  Load— Elasticity  of  Muscle  —  Chemical 
Changes  Accompanying  Contraction — Rigor  Mortis 300 

Chapter  XXXII. 

REPRODUCTION. 

Fertilization— The  Accessory  Phenomena  of  Reproduction  in 
Man— Female  Organs— Male  Organs— Impregnation— Ovulation 
— Pregnancy — Birth  • 303 

APPENDIX. 
Fundamental   Demonstrations  in   Physiology 309 


ILLUSTRATIONS. 

Fig.  Page 

1.  Dialyser    27 

2.  Cells  of  parotid  gland  showing  zymogen  granules 40 

3.  The  nerve  supply  of  the  submaxillary  gland 41 

4.  The  changes  which  take  place  in  the  position  of  the  root  of 

the  tongue,  the  soft  palate,  the  epiglottis  and  the  larynx 

during  the  second  stage  of  swallowing 55 

5  Diagrams  of  outline  and  position  of  stomach  as  indicated  by 
skiagrams  taken  on  man  in  erect  position  at  intervals 
after  swallowing  food  61 

6.  Diagram  of   stomach   showing  miniature   stomach   separated 

from  main  stomach  by  a  double  layer  of  mucous  membrane     62 

7.  Diagram  of  time  it  takes  for  a  capsule  containing  bismuth 

to  reach  the  various  parts  of  the  large  intestine ■. . . .     80 

8.  Diagram  of  Atwater-Benedict  Respiration  Calorimeter 86 

9.  Dietetic  chart  (colored  plate)   104 

10.  Cretin,  19  years  old 126 

11.  Case   of  myxcedema   127 

12.  Before  and  after  onset  of  acromegalis  symptoms 132 

13.  Thomas-Zeiss  Haemocytometer  142 

14.  Diagram  of  circulation  (colored  plate)  158 

15.  Position  of  the  heart  in  the  thorax 160 

16.  Generalized  view  of  the  vertebrate  heart 161 

17.  Diagram  of  valves  of  heart 162 

18.  Dissection  of  heart  to  show  auriculo-ventricular  bundle 165 

19.  Relative  pressure  in  auricle,  ventricle  and  aorta 168 

20.  Diagram  of  experiment  to  show  how  a  pulse  comes  to  disap- 

pear when  fluid  flows  through  an  elastic  tube  when  there 

is  resistance  to  the  outflow 173 

21.  Apparatus  for  taking  tracing  of  the  blood  pressure 174 

22.  Apparatus  for  measuring  the  arterial  blood  pressure  in  man. .  176 

23.  Jacquet   Sphygmocardiograph   181 

24.  Pulse  tracing  made  by  sphygmograph 182 

25.  Effect  of  stimulating  vagus  and  sympathetic  nerves  on  the 

frog's  heart 185 

26.  Tracings  of  arterial  blood  pressure  ,.•♦. 186 

27.  Curve  chart  203 

28.  Diagram    of    structure    of    lungs,    showing    larynx,    bronchi, 

bronchioles  and  alveoli 207 

29.  The  position  of  the  lungs  in  the  thorax 209 

xiv 


ILLUSTRATIONS.  XV 

Fig.  Page 

30.  Hering's  apparatus  for  demonstrating  the  action  of  the  respir- 

atory  pump 210 

31.  Diagram  to  show  movement  of  diaphragm  during  respiration  211 

32.  Position  to  be  adopted  for  effecting  artificial  respiration 215 

33.  Diagram  of  laryngoscope 22.5 

34.  Position  of  the  glottis  preliminary  to  the  utterance  of  sound.  .   226 

35.  Position   of  open   glottis   226 

36.  The  position  of  the  tongue  and  lips  during  the  utterance  of 

the  letters   indicated   228 

37.  Diagram  of  the  uriniferous  tubules,  the  arteries  and  the  veins 

of  the  kidney  (colored  plate)   232 

38.  Diagram  of  urinary  system  236 

39.  Schema  of  simple  reflex  arc  240 

40.  Diagram  of  nervous  system  of  segmented  invertebrate 242 

41.  The  simplest  reflex  arc  in  the  spinal  cord 244 

42.  Diagram  of  section  of  spinal  cord,  showing  tracts 247 

43.  Reflex  arc  through  the  spinal  cord,  in  v^^hich  an  intermediary 

neurone  exists  between  the  afferent  and  efferent  neurones 
(colored    plate)    247 

44.  Course  of  the  pyramidal   fibers  from  the  cerebral  cortex  to 

the  spinal  cord   (colored  plate) 248 

45.  Under  aspect  of  human  brain 257 

46.  Vertical  transverse  section  of  human  brain 258 

47.  Diagram  of  the  dorsal  aspect  of  the  medulla  and  pons,  show- 

ing  the   floor   of   the  fourth   ventricle   with   the   nuclei    of 
origin  of  the  cranial  nerves  (colored  plate)   260 

48.  Diagram   to   show   areas   of   referred    pain   in   distribution   of 

fifth   nerve   due   to   affections   of   the    various   teeth    (front 
view)    (colored   plate)    262 

49.  Diagram   to   show   areas   of   referred    pain    in    distribution   of 

fifth   nerve    due   to    affections    of    the    various    teeth    (side 
view)    (colored   plate)    264 

50.  Cortical   centers  in   man   270 

51.  The  semicircular  canals  of  the  ear,   showing  their  arrange- 

ment in  the  three  planes  of  space 276 

52.  Formation  of  image  on  retina 281 

53.  Section  through  the  anterior  portion  of  the  eye 282 

54.  A,  spherical  aberration;    B,  chromatic  aberration 285 

55.  Errors   in    refraction    286 

56.  Semidiagrammatic  section  through  the  right  ear 292 

57.  Diagrammatic  view  of  the  organ  of  Corti  (colored  plate) 292 

58.  Tympanum  of  right  side  with  the  auditory  ossicles  in  place..  294 

59.  Showing  course  of  taste  fibers  from   tongue   to   brain 296 


HUMAN  PHYSIOLOGY 


CHAPTER  I. 
THE  CHEMICAL  BASIS  OF  THE  CELL. 

The  Scope  of  Physiology. — Physiology  is  the  study  of  the 
phenomena  of  living  things,  just  as  anatomy  or  morphology  is  a 
study  of  their  structure.  The  study  of  anatomy  is  most  logically 
pursued  hy  starting  with  the  simplest  organisms  and  gradually 
proceeding  through  the  more  complex  forms  until  man  is 
reached.  Except  for  certain  fundamental  functions,  such  as 
nutrition,  which  are  common  to  all  cells,  this  method  is  not 
the  most  suitable  one  to  pursue  in  physiology,  because  in  the  low- 
est organisms  all  of  the  functions  are  crowded  together  in  a  lim- 
ited number  of  cells — indeed,  it  may  be  in  one  single  cell.  It  is 
easier  to  study  a  function  when  it  is  performed  by  a  tissue  or 
organ  that  has  been  set  apart  for  this  particular  purpose  than 
when  it  is  performed  by  cells  that  do  many  other  things.  Another 
reason  for  paying  more  attention  to  the  functions  of  higher 
rather  than  lower  animals  is  that  the  knowledge  which  we  acquire 
may  be  more  directly  applicable  in  explaining  the  functions  of 
man,  and  therefore  in  enabling  us  more  readily  to  detect  and 
rectify  any  abnormalities. 

During  the  embryonic  development  of  one  of  the  higher  ani- 
mals, a  single  cell,  the  ovum,  produces  numerous  other  cells, 
which  become  more  and  more  collected  into  groups,  in  many  of 
which  the  cells  undergo  very  marked  changes  in  shape  and 
structure,  or  produce  materials,  such  as  the  skeleton  or  teeth, 
which  show  no  cell  structure  whatsoever.  Thus  we  have  formed 
the  tissues  and  organs,  each  having  some  particular  function  of 

17 


18  HUMAN   PHYSIOLOGY. 

its  own,  although  certain  functions  remain  which  are  common 
to  all.  In  other  words,  as  the  organism  becomes  more  and  more 
complex,  there  comes  to  be  a  division  of  labor  on  the  part  of  the 
cells  that  comprise  it.  The  conditions  are  exactly  like  those 
which  obtain  in  the  development  of  a  community  of  men.  In 
primeval  communities  there  is  little  division  of  labor,  every  indi- 
vidual makes  his  own  clothes,  hunts  his  own  food,  manufactures 
and  uses  his  own  implements  of  war,  but  as  civilization  begins 
to  appear,  certain  individuals  specialize  as  hunters  and  fighters, 
others  as  makers  of  clothing,  others  as  artisans.  Although,  in 
its  first  stages,  this  division  of  labor  may  be  far  from  absolute, 
for  every  member  of  the  community  must  still  fight  and  take  part 
in  the  building  of  his  hut,  yet  it  soon  tends  to  become  more  and 
more  so,  until,  as  in  the  civilized  communities  of  this  twentieth 
century  of  ours,  specialization  has  become  the  order  of  the  day. 

A  good  example  of  a  one-celled  animal  is  the  amoeba,  which  is 
often  found  floating  in  stagnant  water,  and  which  consists  of 
nothing  more  than  a  mass  of  tissue,  or  protoplasm,  as  it  is  called, 
and  yet  this  apparently  simple  structure  can  move  from  place  to 
place,  it  can  pick  up  and  incorporate  with  its  own  substance  par- 
ticles of  food  with  which  it  comes  in  contact,  it  can  store  up  as 
granules  certain  of  these  foodstuffs,  and  get  rid  of  others  that  it 
does  not  require ;  it  grows  as  a  result  of  this  incorporation,  until 
at  last  it  splits  in  two  and  each  half  repeats  the  cycle.  In  other 
words,  this  single  cell  shows  all  of  the  so-called  attributes  of  life : 
movement,  digestion  and  assimilation  of  food,  growth  and  repro- 
duction. No  one  of  these  properties  is  necessarily  confined  to 
living  structures  alone,  for  some  perfectly  inanimate  bodies  may 
exhibit  one  or  other  of  them,  yet  when  all  occur  together,  we 
consider  the  structure  to  be  living. 

In  the  higher  animals,  these  functions  are  performed  by  the 
so-called  systems,  such  as  the  digestive,  the  circulatory,  the  res- 
piratory, the  excretory,  the  motor,  the  nervous  and  the  reproduc- 
tive, each  system  being  composed  of  certain  organs  and  tissues 
which  are  designed  for  the  special  purpose  of  carrying  out  some 
particular  function  or  functions.  One  function,  however,  is  com- 
mon to  all  of  the  organs  and  tissues,  namely,  that  of  nutrition, 


THE  CHEMICAL  BASIS  OP  THE  CELL.  19 

which  includes  the  process  by  which  the  digested  food  is  built  up 
into  the  protoplasm  of  the  cells,  or  assimilation,  and  that  by 
which  the  resulting  substances  are  broken  down  again,  or  disas- 
similation.  It  is  by  these  processes  that  the  energy  of  life  is  set 
free;  the  energy  by  which  the  tissues  perform  their  functions, 
and  which  appears  as  body  heat.  Every  cell  in  the  animal  body 
is  therefore  a  seat  of  energy  production,  and  at  the  same  time 
each  is  a  machine  for  converting  this  energy  into  some  definite 
form  of  work.  In  this  regard  the  animal  machine  differs  from  a 
steam  engine,  in  which  energy  liberation  occurs  in  the  furnace, 
and  conversion  of  this  energy  to  movement  occurs  in  the  pis- 
tons. The  furnace  and  the  machinery  of  the  animal  body  are 
located  in  the  tissue  cells,  and  the  digestive,  circulatory,  respira- 
tory and  excretory  systems  are  provided  for  the  purpose  of 
transporting,  to  and  from  the  living  cells,  the  fuel  (i.  e.,  the 
food),  along  with  the  oxygen  to  burn  it  and  the  gases  produced 
by  its  combustion.  These  processes  of  assimilation  and  disas- 
similation  constitute  the  study  of  metabolism,  the  practical  side 
of  which  is  included  in  the  science  of  nutrition. 

The  Physico-Chemical  Basis  of  Life. 

With  the  object  of  ascertaining  to  what  extent  the  known  laws 
of  physics  and  chemistry  can  explain  the  fundamental  processes 
that  are  common  to  all  cells,  we  must  make  ourselves  familiar, 
first  of  all,  with  the  chemical  and  physical  nature  of  the  constitu- 
ents of  the  cell,  and  secondly  with  the  physico-chemical  laws 
which  govern  the  reactions  that  take  place  between  these  con- 
stituents. The  same  laws  will  control  the  reactions  which  take 
place  in  the  juices  secreted  by  cells;  for  example,  in  the  blood 
and  in  the  secretions,  such  as  the  saliva. 

The  Chemical  Basis  of  Animal  Tissues. — Certain  substances 
are  found  in  every  living  cell  and  in  approximately  equal  quan- 
tities ;  hence  these  may  be  considered  the  primary  constituents  of 
protoplasm.  In  general  they  consist  of  the  proteins,  lipoids,  in- 
organic salts,  water,  and  probably  the  carbohydrates.  Protoplasm 
is  the  substance  composed  of  these  primary  constituents.    By  its 


20  HUMAN   PHYSIOLOGY. 

activity  the  protoplasm  produces  the  secondary  constituents  of 
the  cell,  which  are  not  the  same  in  all  cells,  and  which  include  the 
granules  of  pigment  or  other  material,  the  masses  of  glycogen, 
the  globules  of  fat  or  the  vesicles  of  fluid  which  are  found  em- 
bedded in  the  protoplasm. 

By  whatever  process  we  attempt  to  isolate  its  constituents,  we 
of  course  kill  the  cell,  so  that  we  can  never  learn  by  analysis  what 
may  have  been  the  real  manner  of  union  of  these  substances  in 
the  living  condition.  All  we  can  find  out  is  the  nature  of  the 
building  material  after  the  structure  (the  cell)  into  which  it  is 
built  has  been  pulled  to  pieces..  If  the  chemical  process  by  which 
we  disintegrate  the  cell  is  a  very  energetic  one,  for  example,  com- 
bustion, we  always  find  the  elements,  carbon,  hydrogen,  nitrogen, 
oxygen,  sulphur,  phosphorus,  sodium,  potassium,  calcium,  chlo- 
rine, and  usually  traces  of  other  elements,  such  as  iodine,  iron, 
etc.  If  the  decomposition  be  less  complete,  definite  chemical 
compounds  are  obtained,  namely,  water,  proteins,  lipoids,  car- 
bohydrates, and  the  phosphates  and  chlorides  of  sodium,  potas- 
sium and  calcium.  We  shall  proceed  to  consider  briefly  the  main 
characteristics  of  each  of  these  substances  and  their  place  in  the 
animal  economy. 

Water. — This  is  the  principal  constituent  of  active  living 
organisms,  and  is  the  vehicle  in  which  the  absorbed  foodstuffs 
and  the  excretory  products  are  dissolved.  It  may  be  said  indeed 
that  protoplasm  is  essentially  an  aqueous  solution,  in  which  other 
substances  of  vast  complexity  are  suspended.  Water,  on  account 
of  its  very  unique  physical  and  chemical  properties,  is  of  prime 
importance  in  all  physiological  reactions.  These  properties  are: 
its  chemical  inactivity  at  body  temperatures;  its  great  solvent 
power  (it  is  the  best  known  universal  solvent)  ;  its  specific  heat, 
or  capacity  of  absorbing  heat ;  and,  depending  on  this,  the  large 
amount  of  heat  which  it  takes  to  change  water  into  a  vapor — 
latent  heat  of  steam.  These  last  mentioned  properties  are  made 
use  of  in  the  higher  animals  for  regulating  the  body  temperature. 

Of  great  importance  in  the  maintenance  of  the  chemical  bal- 
ance of  the  body  are  the  electric  phenomena  which  attend  the 
solution  of  certain  substances  in  water.     This  will  be  discussed 


THE  CHEMICAL  BASIS  OF  THE  CELL.  21 

later  in  connection  with  ionization.  Water  has  also  a  very  great 
surface  tension.  It  is  this  property  which  determines  the  height 
to  which  water  will  rise  in  plants  and  in  the  soil,  and  which  no 
doubt  plays  a  role  in  the  processes  of  absorption  going  on  in 
various  parts  of  the  animal  body. 

Proteins. — The  great  importance  of  proteins  in  animal  life  is 
attested  by  the  fact  that  they  are  absolutely  indispensable  in- 
gredients of  food.  An  animal  fed  on  food  containing  no  protein 
will  die  nearly  as  soon  as  if  food  had  been  withheld  altogether. 
Proteins  are  complex  bodies  composed  of  carbon,  hydrogen,  oxy- 
gen, nitrogen,  and,  in  nearly  all  cases,  sulphur.  Some  may  con- 
tain in  addition  phosphorus,  iron,  iodine,  or  certain  other 
elements.  The  proportions  in  which  the  above  elements  are 
found  in  different  proteins  do  not  vary  so  much  as  the  differences 
in  the  chemical  behavior  of  the  proteins  would  lead  us  to  expect. 
In  general  the  percentage  composition  by  weight  is: 

Carbon 53    per  cent 

Hydrogen 7    per  cent 

Oxygen 22    per  cent 

Nitrogen  16    per  cent 

Sulphur  .'. 1  to  2  per  cent 

The  essential  differences  in  the  structure  of  the  molecules  of 
different  proteins  have  been  brought  to  light  by  studies  of  the 
products  obtained  by  partially  splitting  up  the  molecule.  We 
are  able  to  do  this  by  subjecting  protein  to  the  action  of  super- 
heated steam,  or  by  boiling  with  acids  or  alkalies  in  various  con- 
centrations, or  by  the  action  of  the  ferments  of  digestive  juices 
or  by  bacteria.  The  cleavage  produced  by  ferments  or  bacteria 
is  much  more  discriminate  than  that  brought  about  by  strong 
chemical  reagents ;  that  is  to  say,  the  chemical  groupings  are  not 
so  roughly  torn  asunder  by  the  biological  as  by  the  chemical 
agencies. 

At  jBrst  the  proteins  break  up  into  compounds  still  possessing 
many  of  the  features  of  the  protein  molecule.  These  are  the 
proteoses  and  peptones,  which  consist  of  aggregates  of  smaller 


22  HUMAN  PHYSIOLOGY. 

molecules,  capable  of  being  further  resolved  into  simple  crystal- 
line substances.  These  have  been  called  the  building  stones  of  the 
protein  molecule,  and  although  they  differ  from  one  another  in 
many  respects,  they  have  one  feature  in  common,  namely,  that 
each  consists  of  an  organic  acid  having  one  or  more  of  its  hydro- 
gen atoms  substituted  by  the  radicle,  NHg.  Such  substances  are 
called  amino  todies  or  amino  acids.  For  example,  the  formula  of 
acetic  acid  is  CH...COOH.  If  for  one  of  the  H  atoms  there  is  sub- 
stituted the  NH„  group,  we  have  CH0NH2COOH,  which  is  amino 
acetic  acid,  or  glycocoll.  The  same  sort  of  substitution  may  take 
place,  not  alone  in  the  simple  organic  acids  containing  one  acid 
group,  but  also  in  those  containing  two  acid  groups,  as  in  amino- 
succinic  acid,  COOH.  CHgfNHJCOOH,  or  in  acids  containing 
the  aromatic  or  benzene  ring  group,  as  in  the  case  of  tyrosine, 
CgH^OH.  C2H3.  NHoCOOH,  or  again  there  may  be  two  amino 
acid  groups  present,  as  in  the  diamino  acid,  ornithin  or  diamino- 
valeric  acid,  C.H.CNHJXOOH. 

That  the  large  and  complex  protein  molecule  is  really  built  up 
out  of  these  amino  bodies  has  been  very  conclusively  shown  by 
Emil  Fischer,  who  succeeded  in  causing  two  or  more  of  them  to 
become  united  to  form  a  body  called  a  polypeptid.  When  several 
amino  bodies  were  thus  synthesized,  the  polypeptid  was  found  to 
possess  many  of  the  properties  of  peptones,  which  we  have  just 
stated  are  the  earliest  decomposition  products  of  protein. 

Proteins  differ  from  one  another,  not  only  in  the  nature  of  the 
amino  bodies  of  which  they  are  composed-  (although  certain  of 
these  are  common  to  all  proteins),  but  also  in  the  manner  in 
which  the  amino  bodies  are  linked  together.  We  shall  see  the 
practical  value  of  knowing  what  are  the  amino  bodies  in  a  given 
protein  when  we  come  to  the  subject  of  dietetics  (see  p.  99). 

The  proteins  of  the  cell  are  classified  into  two  groups.  The 
first  includes  the  simple  proteins,  such  as  egg  and  serum  albumin ; 
and  the  second,  the  compound  proteins,  from  which  non-protein 
groups  can  be  split  off.  As, primary  cell  constituents,  the  follow- 
ing simple  and  compound  proteins  are  important:  albumin, 
globulin,  nucleoprotein,  and  the  glycoproteins.  They  are  all  of 
the  nature  of  colloidal  substances  (see  p.  32),  and  therefore  are 


THE  CHEMICAL  BASIS  OF  THE  CELL.  23 

either  precipitated  or  coagulated  when  solutions  containing  them 
are  boiled  or  have  inorganic  salts  dissolved  in  them. 

Albumins  are  characterized  chiefly  by  their  great  solubility  in 
water.  Three  forms  are  of  importance :  egg  albumin,  lactal- 
bumin  of  milk,  and  serum  albumin. 

Globulins  occur  principally  in  the  muscle  proteins,  and  are 
insoluble  in  Avater,  but  soluble  in  dilute  neutral  salt  solutions. 
Many  consider  that  the  albumins  and  globulins  are  only  nutri- 
tive materials  out  of  which  the  protoplasm  manufactures  the 
compound  proteins,  these  being  the  essential  proteins  of  the  cell. 

Nucleo proteins,  both  in  quantity  and  in  relation  to  their  activ- 
ity, are  probably  the  most  important  constituents  of  the  cell. 
They  have  a  very  complex  structure,  and  occur  in  many  varieties. 
They  consist  of  a  combination  between  protein  and  a  substance 
called  nucleic  acid,  which,  on  being  broken  up  by  chemical 
means,  yields  phosphoric  acid,  a  simple  sugar  called  pentose,  and 
nitrogenous  substances  known  as  purin  bases,  and  pyrimidines. 
The  purine  bases  are  of  great  interest,  because  they  are  the  ante- 
cedents in  the  body  of  uric  acid,  which,  being  relatively  insoluble, 
may  become  deposited  from  the  body  fluids  and  cause  gout  or 
gravel.  That  it  is  possible  to  have  an  enormous  variety  of  nucleo- 
proteins  can  be  imagined  when  we  consider  that  there  exist  differ- 
ent sort  of  purin  bases,  of  carbohydrates,  and  of  amino  bodies. 
The  nucleus  of  the  cell  contains  a  nucleoprotein  which  is  particu- 
larly rich  in  purin  bases  and  is  often  called  nuclein. 

Phosphoproteins  are  compounds  of  phosphoric  acid  and  simple 
proteins,  without  any  nucleic  acid.  An  example  is  the  casein  of 
milk  (see  p.  105).  * 

Glycoproteins  are  compound  of  carbohydrates  with  proteins. 
The  mucin  of  saliva  is  an  example  (see  p.  46). 

Insoluble  proteins  resemble  the  coagulated  proteins,  and  are 
left  behind  after  the  extraction  of  the  other  proteins  from  the 
cell.  .''"I^ 

Lipoids. — These  include  all  the  substances  composing  a  cell 
which  are  soluble  in  fat  solvents.  Besides  fats  and  fatty  acids, 
the  most  important  of  these  substances  are  lecithin  and  choles- 
terol. 


24  HUMAN   PHYSIOLOGY, 

Lecithin  is  widely  distributed  in  the  animal  body,  and  is  very 
important  in  the  metabolism  and  in  the  physical  structure  of  the 
cell.  It  consists  chemically  of  glycerine,  fatty  acid,  phosphoric 
acid,  and  a  nitrogenous  base  called  cholin. 

Cholesterol  is  another  widely  distributed  lipoid.  It  is  not  in 
reality  a  fatty  body,  but  rather  resembles  the  terpenes.  Lecithin 
and  cholesterol  are  abundant  in  brain  tissue,  in  the  envelopes  of 
erythrocytes,  and  in  bile. 

The  fats  exist  mainly  as  secondary  constituents  of  the  cell, 
being  deposited  in  very  large  amounts  in  certain  of  the  connective 
tissue  cells  of  the  body,  in  bone  marrow  and  in  the  omental  tis- 
sues. Chemically,  the  tissue  fats  are  of  three  kinds :  olein,  pal- 
mitin,  and  stearin,  each  having  a  distinctive  melting  point.  They 
are  compounds  of  the  tri-valent  alcohol,  glycerine,  and  one  of  the 
higher  fatty  acids,  oleic,  palmitic,  or  stearic  acid.  Besides  those 
that  are  present  in  the  animal  tissues,  fats  made  up  of  glycerine 
combined  with  various  lower  members  of  the  fatty  acid  series 
occur  in  such  secretions  as  milk.  In  order  to  understand  the 
influence  which  fats  have  on  general  metabolism,  it  is  important 
to  remember  that  they  differ  from  the  carbohydrates  in  contain- 
ing a  very  low  percentage  of  oxygen  and  a  relatively  high  per- 
centage of  hydrogen  and  carbon.  Thus,  the  empirical  formula 
of  palmitin  is  CgiHgsOg  or  C3lIg(Ci6H3i02)3,  that  of  dextrose 
CeH^^.Oe?  and  of  protein  C^oHiiaNigOaaS. 

The  Carbohydrates  are  also  mainly  secondary  cell  constitu- 
ents, although  it  is  becoming  more  and  more  evident  that  they 
are  also  necessary  as  primary  constituents.  In  general  they  may 
be  defined  chemically  as  consisting  of  the  elements  C,  H,  and  0, 
the  latter  two  being  present  in  the  molecule  in  the  same  propor- 
tion as  in  water ;  thus,  the  formula  for  dextrose  is  CJI^^^q. 

The  basic  carbohydrates  are  the  simple  sugars  or  monosac- 
charides, such  as  grape  sugar  or  dextrose.  When  two  molecules 
of  monosaccharide  become  fused  together  with  the  elimination 
of  a  molecule  of  water  (thus  giving  the  formula  Ci2H220ii)j  a 
secondary  sugar  or  disaccharide  results.  Cane  sugar,  lactose  (or 
milk  sugar)  and  maltose  (or  malt  sugar)  are  examples.  If  sev- 
eral nonsaccharide  molecules  similarly   fuse   together,   polysac- 


THE  CHEMICAL  BASIS  OP  THE  CELL.  25 

charides  having  the  formula  (CcHioOg)^^  are  formed.  These  in- 
clude the  dextrines  or  gums,  glycogen  or  animal  starch,  the  ordi- 
nary starches,  and  cellulose.  Since  so  many  molecules  are  fused 
together,  it  is  not  to  be  wondered  at  that  there  should  be  so  many 
varieties  of  each  of  these  classes  of  polysaccharides,  for,  as  in  the 
case  of  proteins,  not  only  may  the  actual  "building  stones"  of 
the  molecule  be  different,  but  they  may  be  built  together  in  very 
diverse  ways.  The  polysaccharides  may  be  hydrolyzed  (i.  e., 
caused  to  take  up  water  and  split  up)  into  disaccharides,  and 
these  into  monosaccharides  by  boiling  with  acids  or  by  the  action 
of  diastatic  and  inversive  ferments  (see  p.  36). 

The  following  formulae  illustrate  these  facts: 

1.  C(5Hj206=a  monosaccharide  (dextrose). 

2.  Ci2H2oOii::=  a  disaccharide  (cane  sugar)  composed  of: 
CeHoOe  +  CeHj.Oe— H.O. 

3.  {Q^^^O^)n  =  a  polysaccharide  (starch)  composed  of: 
n  CgHjoOe  —  n  HgO  where  n  signifies  that  an  indefinite  number 
of  molecules  are  involved  in  the  reaction. 


CHAPTER  11. 

THE    INFLUENCE    OF   PHYSICO-CHEMICAL    LAWS    ON 
PHYSIOLOGICAL  PROCESSES:    ENZYMES. 

Having  learned  of  what  materials  the  cell  is  composed,  we  may 
proceed  to  enquire  into  the  chemical  and  physical  reactions  by 
which  it  performs  its  functions.  The  cell,  erither  of  plants  or 
of  animals,  may  be  considered  as  a  chemical  laboratory,  in  which 
are  constantly  going  on  reactions,  that  are  guided,  as  to 
their  direction  and  scope,  by  the  physical  conditions  under  which 
they  occur.  A  study  of  the  material  outcome  of  these  reactions 
constitutes  the  science  of  metabolism,  to  which  special  chapters 
are  devoted  further  on.  At  present,  however,  we  must  briefly 
examine  the  physico-chemical  conditions  existing  in  the  cell 
which  may  give  the  directive  influence  to  the  reactions.  Why 
should  certain  cells,  like  those  which  line  the  intestine,  absorb 
digested  food  and  pass  it  on  to  the  blood,  whilst  others,  like  those 
of  the  kidney,  pick  up  the  effete  products  from  the  blood  and 
excrete  them  into  the  urine?  We  must  ascertain  whether  these 
are  processes  depending  on  purely  physico-chemical  causes,  or 
whether  they  are  a  function  of  the  living  protoplasm  itself,  a 
vital  action,  as  we  may  call  it.  In  general  it  may  be  said  that 
the  aim  of  most  investigations  of  the  activities  of  cells  is  to  find 
a  physico-chemical  explanation  for  them,  and  it  is  one  of  the 
achievements  of  modern  physiology  that  some  should  have  been 
thus  explainable.  A  large  number,  however,  do  not  permit  of 
such  an  explanatic-n,  and  this  has  induced  certain  investigators 
to  believe  that  there  are  some  animal  functions  which  are  strictly 
vital  and  can  never  be  accounted  for  on  a  physical  basis.  The 
"physical"  and  the  "vital  schools"  of  physiologists  are  there- 
fore always  with  us. 

From  the  standpoint  of  physical  chemistry,  the  cell  may  be 
considered  as  a  collection  of  two  classes  of  chemical  substances, 

26 


CRYSTALLOIDS. 


27 


called  crystalloids  and  colloids,  dissolved  in  water,  or  in  the  lip- 
oids, or  in  each  other,  and  surrounded  by  a  membrane  which  is 
permeable  towards  certain  substances  but  not  towards  others 
(semipermeable,  as  it  is  called).  On  a  larger  scale,  the  same  gen- 
real  conditions  exist  in  all  of  the  animal  fluids,  such  as  the  blood, 
the  lymph,  the  secretions  and  the  excretions.  We  may  therefore 
study  the  above  laws  with  a  view  to  applying  them  to  both  cells 
and  body  fluids. 

Properties  of  Crystalloids. — As  their  name  implies,  these 
form  crystals  under  suitable  conditions.  When  present  in  solu- 
tion they  diffuse  quickly  thi-oughout  the  solution,  and  can  readily 


Fig.  1. — Dlalyser  made  of  lube  of  parchment  paper  suspended  in  a  vessel 
of  distilled  water.  The  fluid  to  be  dialysed  is  placed  in  the  tube,  and  the 
distilled  water  must  be  frequently  changed. 


pass  through  membranes,  such  as  a  piece  of  parchment,  placed 
between  the  solution  containing  them  and  another  solution.  This 
process  is  called  dialysis,  and  the  apparatus  used  for  observing 
it,  a  dialyser  (see  Fig.  1).  Dialysis  differs  from  filtration,  the 
latter  process  consisting  in  the  passage  of  fluids,  and  the  sub- 
stances dissolved  in  them,  through  more  or  less  pervious  mem- 
branes as  a  result  of  differences  of  pressure  on  the  two  sides  of 
the  membrane.  If  instead  of  using  a  simple  membrane,  such  as 
parchment,  we  choose  one  which  does  not  permit  the  crystalloid 
itself  to  diffuse,  but  permits  the  solvent  to  do  so — a  semipermeable 
membrane,  as  it  is  called, — a  very  interesting  property  of  dis- 
solved crystalloids  comes  to  light,  namely,  their  tendency  to  oc- 


28  HUMAN  PHYSIOLOGY. 

cupy  more  room  in  tjie  solvent,  that  is,  to  cause  dilution  by  at- 
tracting the  solvent  through  the  membrane.  Cell  membranes  are 
semipermeable,  but  they  are  too  small  and  delicate  for  most  ex- 
perimental purposes.  For  this  purpose  we  use  an  artificial  mem- 
brane composed  of  a  precipitate  of  copper  ferrocyanide  sup- 
ported in  the  pores  of  an  unglazed  clay  vessel.  If  a  solution  of 
crystalloid — say,  cane  sugar — be  placed  in  such  a  semipermeable 
membrane  and  this  then  submerged  in  water,  it  will  be  found 
that  the  cane  sugar  solution  quickly  increases  in  volume,  or  if 
expansion  be  impossible,  a  remarkably  high  pressure  will  be 
developed.  This  is  called  osmotic  pressure,  and  it  is  a  measure 
of  the  tendency  of  dissolved  crystalloids  to  expand  in  the  solvent. 

It  has  been  found  that  the  laws  which  govern  osmotic  pressure 
are  identical  with  tliose  governing  the  behavior  of  gases.  There- 
fore, osmotic  pressure  ought  to  be  proportional  to  the  number  of 
molecules  of  dissolved  crystalloid.  This  is  the  case  for  the  sugars, 
but  it  is  not  so  for  the  saline  crystalloids,  such  as  the  alkaline 
chlorides,  nitrates,  etc.,  for  these  cause  a  greater  osmotic  pres- 
sure than  we  should  expect  from  their  molecular  weights.  Why 
is  this?  The  answer  is  revealed  by  observing  the  behavior  of 
the  two  classes  of  crystalloids  towards  the  electric  current.  So- 
lutions of  sugars  or  urea  do  not  conduct  the  current  any  better 
than  water,  whereas  solutions  of  saline  crystalloids  conduct  very 
readily.  The  former  are  therefore  called  non-electrolytes  and 
the  latter  electrolytes.  It  has  been  found  that  the  reason  for 
this  is  that  molecules  of  .electrolytes  when  they  are  dissolved 
break  into  parts  called  "ions,"  each  ion  being  charged  with 
electricity  of  a  certain  sign,  i.  e.,  positive  or  negative.  When- 
ever an  electric  current  is  passed  through  the  solution,  the  ions, 
hitherto  distributed  throughout  the  solution  in  pairs  carrying 
electrical  charges  of  opposite  signs,  now  line  themselves  up  so 
that  the  ions  with  one  kind  of  charge  form  a  chain  across  the 
solution  along  which  that  kind  of  electricity  readily  passes,  and 
in  so  doing  carries  the  ions  with  it. 

This  splitting  of  electrolytes  into  ions  is  called  dissociation  or 
ionization.  The  ions  which  carry  a  charge  of  positive  elec- 
tricity and  which  therefore  travel  towards  the  kathode  or  nega- 


CRYSTALLOIDS.  29 

tive  pole,  (since  unlike  electricities  attract  each  other)  are  called 
katJiions,  and  the  negativelj'  charged  ions  that  travel  to  the  anode, 
anions.  Hydrogen  and  the  metallic  elements  belong  to  the  group 
of  kathions;  oxygen,  the  halogens  and  all  acid  groups,  to  the 
anions.  These  facts  may  be  more  clearly  understood  from  the 
following  equations : 

In  water,  or  in  a  solution  of  a  non-electrolyte,  molecules  of 
HoO  or  non-electrolyte  may  be  represented  as  existing  thus : 

H.O  H2O  H2O 

H.O  H2O  H2O 

H2O  H2O  H2O 

In  a  solution  of  an  electrolyte,  the  molecules  split  into  ions 
thus : 

Na+     CI-    Na*     CI"     Na*     Cl" 

Na^     CI-    Na^     CI"     Na^     Cl" 
Na^     CI-    Na^     CI-     Na^     CV 

When  an  electric  current  passes  through  a  solution  of  an 
electrolyte,  the  ions  arrange  themselves  thus : 

Kathode"  Anode"^ 

Na^  Na^  Na^  CI"  CI-  CI" 
Na^  Na*  Na^  .  CI-  Cb  CI- 
Na^    Na^     Na^     CI-     CI-     Cl- 

To  return  to  osmotic  pressure,  the  ions  influence  this  as  if  they 
were  molecules,  so  that  when  we  dissolve,  say,  sodium  chloride 
in  water,  the  osmotic  pressure  is  almost  twice  what  it  should  be, 
because  every  molecule  has  split  into  two  ions. 

Osmotic  Phenomena  in  Cells. — Over  and  over  again  we  shall 
have  to  refer  to  these  physico-chemical  processes  in  explaining 
physiological  phenomena.  For  the  present  it  may  make  matters 
clearer  if  we  consider  how  osmosis  explains  the  behavior  of  cells 
when  suspended  in  different  solutions.  The  cell  wall  acts  as  a 
semipermeable  membrane.  Thus,  if  we  examine  red  blood  cor- 
puscles suspended  in  different  saline  solutions  under  the  micro- 
scope, we  shall  observe  that  they  shrink  or  crenate  when  the  solu- 


30  HUMAN   PHYSIOLOGY. 

tions  are  strong,  and  expand  and  become  globular  in  shape  when 
these  are  weak.  The  shrinkage  is  due  to  diffusion  of  water  out 
of  the  corpuscle  and  the  swelling,  to  its  diffusion  in;  that  is  to 
say,  in  the  former  case  the  osmotic  pressure  of  the  surrounding 
fluid  is  greater  than  that  of  the  corpuscular  contents  and  vice 
versa  in  the  latter  case.  In  this  way  we  have  a  simple  and  con- 
venient method  of  comparing  the  relative  osmotic  pressure  of  dif- 
ferent solutions.  "When  the  solution  has  a  higher  pressure,  it  is 
called  hypertonic,  when  less,  hypotonic,  when  the  same,  isotonic. 
It  is  evident  that  the  body  fluids  must  always  be  isotonic  with  the 
cell  contents,  and  that  we  must  be  careful  never  to  introduce 
fluids  into  the  blood  vessels  that  are  not  isotonic  with  the  blood. 
A  one  per  cent  solution  of  common  salt  is  almost  isotonic  with 
blood,  and  is  accordingly  used  for  intravenous  or  subcutaneous 
injections,  or  for  washing  out  body  cavities  or  surfaces  lined  with 
delicate  membranes,  such  as  the  conjunctiva  or  nares. 

Reaction  of  Body  Fluids. — Closely  dependent  upon  these 
properties  of  ionization  are  the  reactions  which  determine  the 
acidity  and  alkalinity  of  the  body  fluids.  When  we  speak  of  the 
degree  of  acidity  or  alkalinity  of  a  solution  in  chemistry,  we 
mean  the  amount  of  alkali  or  acid,  respectively,  which  it  is  nec- 
essary to  add  in  order  that  the  solution  may  become  neutral  to- 
wards an  indicator,  such  as  litmus.  This  titrible  reaction  is  how- 
ever a  very  different  thing  from  the  real  strength  of  the  acid  or 
alkali;  for  example,  we  may  have  solutions  of  lactic  and  hydro- 
chloric acids  that  require  the  same  amount  of  alkali  to  neutral- 
ize them,  but  the  hydrochloric  acid  solution  will  have  much  more 
powerful  acid  properties  (attack  other  substances,  taste  more 
acid,  act  much  more  powerfully  as  an  antiseptic,  etc.).  The  rea- 
son for  the  difference  is  the  degree  of  ionization ;  the  strong  acids 
ionize  much  more  completely  than  the  weak.  As  a  result  of  this 
ionization,  each  molecule  of  the  acid  splits  into  H-ions  and  an 
ion  composed  of  the  remainder.  To  ascertain  the  real  acidity 
we  must  therefore  measure  the  concentration  of  H-ions.  (These 
considerations  also  apply  in  the  case  of  alkalies,  only  in  this  case 
OH-ions  determine  the  degree  of  alkalinity.)  This  can  be  done 
accurately  by  measuring  the  speed  at  which  certain  chemical 


REACTION  OF  BODY  FLUIDS.  31 

processes  proceed,  that  depend  on  the  concentration  of  H-ions. 
The  conversion  of  cane  sugar  into  invert  sugar  is  a  good  process 
to  employ  for  measuring  the  speed  of  reaction. 

But  even  this  refinement  in  technique  does  not  enable  us  to 
measure  the  H-ion  concentration — for  now  we  must  use  this  ex- 
pression when  speaking  of  acidity  or  alkalinity — of  such  impor- 
tant fluids  as  hlood  and  saliva,  in  which  there  is  an  extremely 
low  H-ion  concentration.  If  either  of  these  fluids  be  placed  on 
litmus  papers,  the  red  litmus  turns  blue,  but  all  that  this  signifies 
is  that  the  litmus  is  a  stronger  acid  than  those  present  in  blood 
or  saliva,  so  that  it  decomposes  the  bases  with  which  they  were 
combined  and  changes  the  color.  If  we  employ  phenolphthalein, 
which  is  a  much  feebler  acid,  then  blood  serum  reacts  neutral  and 
saliva  often  acid. 

There  are  two  methods  open  to  us  for  measuring  the  H-ion 
concentration  in  such  cases : 

1.  The  Hydrogen  Electrode. — Place  the  fluid  (e.  g.,  blood 
serum  or  saliva),  with  a  platinum  electrode  dipping  into  it,  in 
a  small  vessel  filled  with  hydrogen.  Connect  this  hydrogen 
electrode,  as  it  is  called,  with  a  standard  calomel  electrode  by 
means  of  wires  in  the  course  of  which  are  suitably  arranged 
electrical  instruments  for  the  measurement  of  electromotive 
force.  From  the  difference  in  the  electromotive  force  which  is 
found  to  exist  between  the  hydrogen  and  calomel  electrodes, 
we  can  calculate  the  H-ion  concentration.  This  method  is  being 
employed  for  measuring  the  reaction  of  saliva  in  relationship 
to  its  influence  on  caries  of  the  teeth. 

2.  The  Use  of  StandardizcfL  Indicators. — It  has  been  found 
that  different  indicators  change  color  at  different  H-ion  concen- 
trations. By  measuring  the  H-ion  concentration — by  the  elec- 
trical method — of  solutions  containing  known  proportions  of 
acid  and  alkaline  salts  (such  as  NaHoPO^  and  Na2HP04  or 
NaHCO.^),  and  then  observing  their  behavior  with  different  in- 
dicators, it  has  been  possible  to  evaluate  the  latter  in  terms  of 
the  H-ion  concentration  at  which  they  change  color.  Expressing 
the  results  as  the  fraction  of  a  normal  solution  of  H-ion  at  which 
this  change  occurs,  it  has  been  found  that  paranitro-phenol  turns 


32  HUMAN   PHYSIOLOGY. 

at  about  .000,001  (or  1x10-^),  which  is  the  H-ion  concentration  of 
pure  water,  and  is  therefore  the  most  practical  point  to  choose  as 
indicating  neutrality.  Methyl  red  and  rosolie  acid  also  change 
color  about  this  point.  Phenolphthalein,  on  the  other  hand, 
changes  color  at  a  H-ion  concentration  of  1x10^^,  i.  e.,  it  is  very 
sensitive  towards  acids ;  methyl  orange  changes  at  1x10"*,  i.  e.,  it 
is  relatively  insensitive  towards  acids. 

The  indicators  which  change  color  at  about  the  H-ion'  concen- 
trations found  in  animal  fluids  are  therefore  rosolie  acid,  para- 
nitrophenol  and  methyl  red.  By  comparing  the  color  produced 
by  adding  one  of  these  indicators  to  the  unknown  fluid  with  the 
color  obtained  by  adding  the  same  indicator  to  a  series  of  solu- 
tions containing  varying  but  known  H-ion  concentrations,  we 
can  accurately  tell  the  H-ion  concentration  of  the  unknown  so- 
lution, for  the  H-ion  concentration  of  the  solution  whose  tint 
matches  with  that  of  the  unknown  is  the  H-ion  concentration  of 
the  latter.  The  series  of  standard  solutions  is  made  by  mixing 
varying  proportions  of  acid  and  alkaline  phosphates. 

Before  leaving  this  subject,  it  is  important  to  point  out  that 
the  blood  has  an  H-ion  concentration  which  is  practically  the 
same  as  that  of  water,  i.  e.,  is  as  nearly  neutral  as  it  could  be.  It 
also  has  the  power  of  maintaining  this  neutrality  practically  con- 
stant even  when  large  amounts  of  acid  or  alkali  are  added  to  it. 
Although  saliva  and  some  other  body  fluids  are  not  so  nearly 
neutral  as  blood,  yet  they  can  also  lock  away  much  acid  or 
alkali  without  materially  changing  the  H-ion  concentration.  This 
property  is  due  to  the  fact  that  the  body  fluids  contain  such  salts 
as  phosphates  and  carbonates,  which  exist  as  neutral  and  acid 
salts,  and  can  change  from  the  one  state  to  the  other  without 
greatly  altering  the  H-ion  concentration,  and  yet,  in  so  changing, 
can  lock  away  or  liberate  H-  or  OH-ions.  This  has  been  called 
the  "buffer"  action,  and  is  a  most  important  factor  in  maintain- 
ing constant  the  neutrality  of  the  animial  body. 

Colloids. — These  are  substances  which  do  not  diffuse  through 
membranes  when  they  are  dissolved.  Thus  if  blood  serum  be 
placed  in  a  dialyser  which  is  surrounded  by  distilled  water,  all 


COLLOIDS.  33 

the  crystalloids  will  diffuse  out  of  it,  leaving  the  colloids,  which 
consist  mainly  of  proteins.  The  physical  reason  for  this  failure 
to  diffuse  is  the  large  size  of  the  molecules,  in  comparison  with 
the  small  size  of  those  of  the  crystalloids.  By  causing  a  beam  of 
light  to  pass  through  a  colloidal  solution  and  holding  a  micro- 
scope at  right  angles  to  this  beam,  the  colloidal  particles  become 
evident,  just  as  particles  of  dust  become  evident  in  a  beam  of 
daylight  in  a  darkened  room. 

Filters  can  be  made  of  unglazed  porcelain  impregnated  with 
gelatin  in  which  the  pores  are  so  very  minute  that  colloids  can 
not  pass  through  them,  though  water  and  inorganic  salts  do  so. 
When  blood  serum  is  filtered  through  such  a  filter,  the  filtrate 
contains  no  trace  of  protein.  The  colloidal  molecules  can  very 
readily  be  caused  to  fuse  together,  thus  forming  aggregates  of 
molecules  which  become  so  large  that  they  either  confer  an 
opacity  on  the  solution  or  actually  form  a  precipitate. 

This  fusing  together  of  colloidal  particles  can  be  brought  about 
either  by  adding  certain  neutral  salts  or  by  mixing  with  certain 
other  colloids.  The  explanation  of  these  results  is  as  foltows: 
colloidal  molecules  carry  either  a  positive  or  a  negative  electrical 
cliarge,  and  when  this  is  neutralized,  the  colloidal  molecules  fuse 
together,  i.  e.,  become  aggregated.  This  neutralization  of  elec- 
trical charge  can  be  accomplished  either  by  adding  an  electro- 
lyte, one  of  whose  ions  will  supply  the  proper  electrical  charge, 
oi"  by  a  colloid  having  an  opposite  charge.  Thus  the  SO4  anion 
of  NajSO^,  in  virtue  of  charges  of  negative  electricity  which  it 
carries,  will  very  readily  precipitate  such  a  colloid  as  colloidal 
iron  (ferrum  dialysatum,  U.  S.  P.),  which  is  charged  with  posi- 
tive electricity ;  or  again,  this  colloid  itself  will  readily  precipitate 
arsenious  sulphide,  another  colloid  carrying  a  negative  charge. 
The  physiological  importance  of  these  reactions  lies  in  the  fact 
that  they  probablj^  explain  many  of  the  peculiarities  of  behavior 
of  mixtures  of  different  animal  fluids,  such  as  toxins  and  anti- 
toxins (see  p.  149). 

A  property  of  colloids  which  is  closely  related  to  the  above  is 
that  of  adsorption.  This  means  the  tendency  for  dissolved  sub- 
stances to  become  condensed  or  concentrated  at  the  surface  of 


34  HUMAN   PHYSIOLOGY. 

colloidal  molecules.  An  example  is  the  well  known  action  of 
charcoal  when  shaken  with  colored  solutions.  It  removes  the  pig- 
ment by  adsorbing  it.  Adsorption  is  due  to  surface  tension, 
which  is  the  tension  created  at  the  surface  between  a  solid  and  a 
liquid,  or  between  a  liquid  and  a  gas.  It  is  in  virtue  of  surface 
tension  that  a  raindrop  assumes  a  more  or  less  spherical  shape. 
Since  colloids  exist  as  particles,  there  must  be  an  enormous  num- 
ber of  surfaces  throughout  the  solution,  that  is,  an  enormous  sur- 
face tension.  Now  many  substances,  when  in  solution,  have  the 
power  of  decreasing  the  surface  tension,  and  in  doing  so  it  has 
been  found  that  they  accumulate  at  the  surface,  that  is  to  say, 
in  a  colloidal  solution,  at  the  surface  of  the  colloidal  molecules. 
The  practical  application  of  this  is  that  it  helps  to  explain  the 
physical  chemistry  of  the  cell,  the  protoplasm  of  which  is  a  col- 
loidal solution  containing  among  other  things  proteins  and 
lipoids.  The  lipoids  depress  the  surface  tension  and  therefore 
collect  on  the  surface  of  the  cell  and  form  its  supposed  mem- 
brane, whilst  the  proteins  exist  in  colloidal  solution  inside.  It  is 
possibly  by  their  solvent  action  on  lipoids  that  ether  and  chloro- 
form so  disturb  the  condition  of  the  nerve  cells  as  to  cause  anes- 
thesia. A  knowledge  of  colloidal  chemistry  is  coming  to  be  of 
great  importance  in  physiology. 

General  Nature  of  Enzymes  or  Ferments. 

To  decompose  proteins,  fats  or  carbohydrates  into  simple  mole- 
cules in  the  laboratory  necessitates  the  use  of  powerful  chemical 
or  physico-chemical  agencies.  Thus,  to  decompose  the  protein 
molecule  into  amino  bodies  requires  strong  mineral  acid  and  a 
high  temperature.  In  the  animal  body  similar  processes  occur 
readily  at  a  comparatively  low  temperature  and  without  the  use 
of  strong  chemicals  in  the  ordinary  sense.  The  agencies  which 
bring  this  about  are  the  enzymes  or  ferments.  These  are  all  col- 
loidal substances  (see  p.  32),  so  that  they  are  readily  destroyed 
hy  heat  and  are  precipitated  hy  tlie  same  reagents  as  proteins. 
They  are  capable  of  acting  in  extremely  small  quantities.  Thus, 
a  few  drops  of  saliva  can  convert  large  quantities  of  starch  solu- 
tion into  sugar.    During  their  action,  the  enzymes  do  not  them- 


ENZYMES.  35 

selves  undergo  any  permanent  change,  for  even  after  they  have 
been  acting  for  a  long  time,  they  can  still  go  on  doing  their  work 
if  fresh  material  be  supplied  upon  which  to  act.  These  proper- 
ties are  explained  by  the  fact  tliat  they  act  catalytically,  just  as 
the  oxides  of  nitrogen  do  in  the  manufacture  of  sulphuric  acid. 
That  is  to  say,  they  do  not  really  contribute  anything  to  a  chemi- 
cal reaction,  but  merely  serve  as  accelerators  of  reactions,  which 
however  would  occur,  though  very  slowly,  in  their  absence.  Thus, 
to  take  our  example  of  starch  again,  if  this  were  left  for  several 
j^ears  in  the  presence  of  water,  it  would  take  up  some  of  the  water 
and  split  into  several  molecules  of  sugar  (p.  34).  The  enz^nne 
ptyalin  in  saliva  merely  acts  by  hurrying  up  or  accelerating  the 
reaction  so  that  it  occurs  in  a  few  minutes. 

Enzymes  differ  from  inorganic  eatalysers  in  the  remarkable 
specificity  of  their  action,  there  being  a  special  enzyme  for  prac- 
tically every  chemical  change  that  occurs  in  the  animal  hody. 
Thus,  if  we  act  on  any  of  the  sugars  called  disaccharides  (cane 
sugar,  lactose  and  maltose)  with  an  inorganic  caty lytic  agent, 
such  as  hydrochloric  acid,  they  will  split  up  into  their  constitu- 
ent monosaccharide  molecules,  whereas  in  the  body,  each  disac- 
charide  requires  a  special  or  specific  enzyme  for  itself.  The  en- 
zyme acting  on  one  of  them,  in  other  words,  will  be  absolutely 
inert  towards  the  others.  This  specificity  of  action  is  explained 
by  supposing  that  each  substance  to  be  acted  on  (called  the  sub- 
strat)  is  like  a  lock  to  open  which  the  proper  key  (the  enzyme) 
must  be  fitted. 

Enzymes  are  peculiarly  sensitive  towards  the  chemical  condi- 
tion of  the  fluid  in  which  they  are  acting,  more  particularly  its 
reaction.  Thus  the  enzyme  of  saliva  acts  best  in  neutral  reaction, 
whereas  the  enzyme  of  gastric  juice  acts  only  in  the  presence  of 
acid,  and  those  of  pancreatic  juice,  in  the  presence  of  alkali. 
Enzymes  may  unfold  this  action  either  inside  or  outside  of  the 
cells  which  produce  them.  Thus,  the  enzymes  produced  in  the 
digestive  tract  act  outside  the  gland  cells,  but  the  enzyme  of  the 
yeast  cell  acts  in  the  cell  itself  and  is  never  secreted.  The  former 
are  called  extracellular  enzymes  and  the  latter  intracellular.  The 
activities  of  intracellular  enzymes  are  much  more  liable  to  be 


36  HUMAN  PHYSIOLOGY. 

interfered  with  bj  unfavorable  conditions  than  those  of  extra- 
cellular enzymes.  This  is  because  the  former  become  inactive 
whenever  anything  occurs  to  destroy  the  protoplasm  of  the  cell 
in  which  they  act.  The  living  protoplasm  is  necessary  to  bring 
the  substrat  in  contact  with  them.  On  this  account  enzymes  used 
to  be  classified  into  organized  and  unorganized.  We  know  that 
there  really  is  no  difference  in  the  enzyme  itself ;  the  only  differ- 
ence is  with  regard  to  the  place  of  activity.  The,  cells  that  com- 
pose the  tissues  of  animals  perform  their  various  chemical  activi- 
ties in  virtue  of  the  intracellular  enzymes  which  they  contain. 
These  are,  therefore,  the  chemical  reagents  of  the  laboratory  of 
life.  After  the  animal  dies,  the  intracellular  enzymes  may  go 
on  acting  for  a  time  and  digest  the  cells  from  within.  This  is 
called  autolysis. 

Enzymes  are  classified  into  groups  according  to  the  nature  of 
the  chemical  action  which  they  accelerate.    Thus: 

Hydrolytic  enzymes — cause  large  molecules  to  take  up  water 
and  split  into  small  molecules.  (Most  of  the  digestive 
enzymes  belong  to  this  class.) 

Oxidative  enzymes  (oxydases) — encourage  oxidation. 

Deamidating — remove  NHo  group. 

Coagulative — convert  soluble  into  insoluble  proteins. 

Each  group  is  further  subdivided  according  to  the  nature  of 
the  substrat  on  which  the  enzymes  act;  e.  g.,  hydrolytic  enzymes 
are  subdivided  into  amylolases^ — acting  on  starch;  invertases — 
acting  on  disaccharides ;  proteases — acting  on  proteins;  ureases 
— acting  on  urea,  etc. 

"When  enzymes  are  repeatedly  injected  into  the  blood,  or  under 
certain  other  conditions,  they  have  the  power,  like  toxines,  of 
producing  antienzymes.  As  their  name  signifies,  these  are  bodies 
which  retard  the  action  of  enzymes.  Thus,  if  sonle  blood  serum 
from  an  animal  into  which  trypsin  has  been  injected  for  some 
days  previously  be  mixed  with  a  trypsin  solution,  the  mixture 
will  digest  protein  very  slowly,  if  at  all,  when  compared  with  a 
mixture  of  the  same  amount  of  trypsin  and  protein  (see  also 
p.  78). 


CHAPTER  III. 

DIGESTION. 

Necessity  and  General  Nature  of  Digestion:    Digestion  in  the 

Mouth. 

The  never-ceasing  process  of  combustion  that  goes  on  in  the 
animal  body,  as  well  as  the  constant  wear  and  tear  of  the  tissues, 
makes  it  necessary  that  the  supply  of  fuel  and  of  building  mate- 
rial be  frequently  renewed.  For  this  purpose  food  is  taken.  This 
food  is  composed  of  fats  and  carbohydrates,  which  are  mainly 
fuel  materials,  of  inorganic  salts  and  water,  which  are  neces- 
sary to  repair  the  worn  tissues  and  of  proteins  which  are  both 
fuel  and  repair  materials,  and  are  therefore  the  most  important 
of  the  organic  foodstuffs.  The  blood  transports  the  foodstuffs 
from  the  digestive  canal  to  the  tissues.  In  the  digestive  canal  the 
foodstuffs  are  digested  by  hydrolyzing  enzymes  (see  p.  36), 
which  are  furnished  partly  in  the  secretions  of  the  digestive 
glands  and  partly  from  the  numerous  micro-organisms  that 
swarm  in  the  intestinal  contents.  The  enzymes,  as  we  have  seen, 
are  very  discriminative  in  their  action,  for  not  only  is  the  enzyme 
for  protein  without  action  on  a  fat  or  carbohydrate,  but  each  of 
the  different  stages  in  protein  break-down  requires  its  own  pe- 
culiar enzyme.  It  becomes  necessary  therefore  that  the  enzymes 
be  mixed  with  the  food  in  proper  sequence,  and  to  render  this 
possible  the  digestive  canal  is  found  to  be  divided  into  special 
compartments,  such  as  the  mouth,  the  stomach,  the  small  intes- 
tines, etc.,  each  provided  with  its  own  assortment  of  enzymes 
and  with  some  mechanism  by  which  the  food,  when  it  has  been 
sufficiently  digested,  can  be  passed  on  to  the  next  stage. 

Such  correlation  between  the  different  stages  of  digestion 
necessitates  the  existence,  in  the  different  levels  of  the  gastro- 
intestinal tract,  of  mechanisms  which  are  specially  developed  to 

37 


38  HUMAN   PHYSIOLOGY. 

bring  about  the  right  secretion  at  the  right  time.  These  mech- 
anisms are  of  two  essentially  different  types,  a  nervous  reflex 
control,  and  a  chemical  or  "hormone"  control.  The  nervous  con- 
trol is  exercised  through  a  nerve  center,  which  is  called  into  ac- 
tivity by  afferent  stimuli  proceeding  from  sensory  nerve  endings 
or  receptors  (see  p.  244)  that  are  especially  sensitized  so  as  to 
be  stimulated  by  some  property  of  food  (its  taste  or  smell,  or 
its  consistency  or  chemical  nature).  This  type  of  control 
exists  where  prompt  response  of  the  glandular  secretion  is  impor- 
tant, as  in  the  mouth  and  in  the  early  stages  of  digestion  in  the 
stomach.  The  hormone  control  consists  in  the  action  directly  on 
the  gland  cells  of  substances  which  have  been  absorbed  into  the 
blood  from  the  mucous  membrane  of  the  gastro-intestinal  tract. 
The  production  of  these  substances  depends  upon  the  nature  of 
the  contents  of  the  digestive  tube.  This  is  a  more  sluggish  proc- 
ess of  control  than  the  nervous,  but  it  is  sufficient  for  the  cor- 
relation of  most  of  the  digestive  functions. 

These  considerations  point  the  way  to  the  scheme  which  we 
must  adopt  in  studying  the  process  of  digestion ;  we  must  explain 
how  each  digestive  juice  comes  to  be  secreted,  what  action  it  has 
on  the  foodstuffs,  and  what  it  is,  after  each  stage  in  digestion  is 
completed,  that  controls  the  movement  onward  of  the  food  to 
the  next  stage.  And  when  we  have  followed  each  foodstuff  to 
its  last  stage  in  digestion,  we  may  then  proceed  to  study  the 
means  by  which  the  digested  foodstuffs  are  absorbed  into  the  cir- 
culating fluids,  and  in  what  form  they  are  carried  to  the  tissues. 

On  account  of  the  varying  nature  of  their  food  we  find  that 
the  digestive  system  differs  considerably  in  different  groups  of 
animals.  In  the  omnivora,  such  as  man,  the  digestive  canal  be- 
gins with  the  mouth  cavity,  in  which  the  food  is  broken  up  me- 
chanically and  is  mixed  with  the  saliva  in  sufficient  amount  to 
render  it  capable  of  being  swallowed.  The  saliva,  by  containing 
starch-splitting  ferment,  also  initiates  the  digestive  process.  The 
food  is  then  carried  by  way  of  the  oesoi^hagus  to  the  stomach,  in 
the  near  or  cardiac  end  of  which  it  collects  and  becomes 
gradually  permeated  by  the  acid  gastric  juice.  It  is  then  caught 
up,   portion   by    portion,    by    the    peristaltic    waves    of    the 


SALIVARY    SECRETION.  39 

further  or  pyloric  end  of  the  stomach  and,  after  being  thor- 
oughly broken  down  by  this  movement  and  partially  digested 
by  the  pepsin  of  gastric  juice,  is  passed  on  in  portions  into 
the  duodenum,  where  it  meets  with  the  secretions  of  the  pancreas 
and  liver.  These  secretions,  acting  along  with  auxiliary  juices 
secreted  by  the  intestine  itself,  ultimately  bring  most  of  it  into  a 
state  suitable  for  absorption.  What  the  digestive  juices  leave 
unacted  on  bacteria  attack,  especially  in  the  cascum,  so  that  by 
the  time  the  food  has  gained  the  large  intestine  it  has  been  di- 
gested as  far  as  it  can  be.  In  its  further  slow  movement  along 
the  large  intestine  the  process  of  absorption  of  water  proceeds 
rapidly. 

Disturbances  in  the  digestive  process  may  be  due  not  only  to 
possible  inadequacy  in  the  secretion  of  one  or  other  of  the  diges- 
tive juices,  but  also  to  disturbances  in  the  movem-ents  of  the 
digestive  canal.  Such  disturbances  wall  not  only  prevent  the 
forward  movement  of  the  food  at  the  proper  time,  but,  by  failing 
to  agitate  the  food,  they  will  prevent  its  thorough  admixture 
with  the  digestive  juices,  so  that  the  enzymes  which  these  con- 
tain will  not  become  properly  mixed  with  the  food. 

/  Digestion  in  the  Mouth. 

Salivary  Secretion. — In  the  mouth,  besides  its  preparation 
for  swallowing,  by  mastication,  etc.,  the  food,  mainly  on  account 
of  its  taste  and  smell,  stimulates  sensory  nerve  endings  which, 
by  acting  on  nerve  centers,  set  agoing  several  of  the  digestive 
secretions.  The  first  of  these  is  \he  secretion  of  the  salivary 
glands.  On  account  of  their  ready  accessibility  to  experimental 
investigation,  very  extended  studies  have  been  made  of  the  sali- 
vary glands,  and  from  these  studies  some  of  the  most  important 
physiological  truths,  concerning  the  nature  of  the  nervous  con- 
ti'ol  of  glands  in  general,  have  been  drawn.  Of  the  three  salivary 
glands  in  man,  the  parotid  secretes  a  watery  saliva  usually  con- 
taining the  enzyme,  ptyalin,  and  the  submaxillary  and  subling- 
ual secrete  a  sticky  saliva  containing  mucin,  usually  along  with 
some  ptyalin.    When  the  glands  are  not  secreting,  the  cells  that 


40 


HUMAN   PHYSIOLOGY. 


compose  them  are  engaged  in  preparing  material  to  be  secreted. 
By  microscopical  examination,  this  material  is  seen  in  the  proto- 
plasm of  the  cells  (Fig.  2)  as  granules,  which  are  extremely 
small  in  the  serous  gland  cells,  but  much  larger  in  the  mucous. 
In  both  types  of  gland  the  granules  so  crowd  the  cell  that  the 
nucleus  becomes  indistinct  and  the  cell  itself  much  swollen. 
After  the  gland  has  been  active,  the  granules  disappear,  being 
evidently  discharged  from  the  cell  into  the  duct  of  the  gland. 
The  granules  are  believed  to  represent  the  precursors  of  the 
ptyalin  or  mucin  of  saliva — hence  their  name  of  "zymogen"  or 
"mother  of  ferment"  granules — rather  than  these  substances 


A.  B. 

Fig.  2. — Cells  of  parotid  gland  showing  zymogen  granules :  A,  after  pro- 
longed rest ;  B,  after  a  moderate  secretion ;  Cj  after  prolonged  secretion. 
(Langley. ) 


themselves.  Watery  or  saline  extracts  of  the  glands  contain 
neither  mucin  nor  ptyalin,  nor  does  the  addition  of  acetic  acid  to 
a  mucous  gland  cause  any  precipitate  of  mucin;  indeed,  it  has 
an  entirely  opposite  action,  it  causes  the  granules  to  swell. 

The  Nerve  Supply  of  the  Salivary  Glands. — The  nerve  fibers 
supplying  the  glands  are  of  the  autonomic  or  visceral  type  (see 
p.  277),  and  they  include  sympathetic  and  cerebro-spinal  fibers. 
The  sympathetic  fibers  are  derived  from  cells  in  the  lateral  horns 
of  the  spinal  cord,  from  which  they  emerge  by  the  upper  three 
or  four  thoracic  roots,  and  after  ascending  as  meduUated  fibers 
in  the  cervical  sympathetic,  terminate  as  synapses  around  the 
cells  of  the  superior  cervical  ganglion.  The  axons  of  these  cells 
proceed  as  non-meduUated  post-ganglionic  fibers  along  the  near- 
est vessels  to  the  respective    glands.     The    cerebral    autonomic 


SALIVARY    SECRETION. 


41 


fibers  arise  from  a  center  in  the  medulla  and  proceed  to  the 
glands  by  various  routes;  those  to  the  submaxillary  and  sub- 
lingual glands  in  the  chorda  tympani,  and  those  to  the  partoid 
by  way  of  the  tympanic  branch  of  the  glosso-pharyngeal.  The 
ganglion  cells  connected  with  the  cerebral  fibers  are  situated 
more  or  less  peripherally;  in  the  case  of  the  submaxillary 
they  are  embedded  in  the  substance  of  the  gland ;  in  the  ease  of 
the  sublingual  gland,  in  the  connective  tissue  of  the  so-called 
submaxillary  triangle,  and  in  the  case  of  the  parotid,  in  the  otic 
ganglion  (Fig.  3). 

In  both  cerebral  and  sympathetic  nerves  there  are  two  vari- 
eties of  fibers,  the  one  vasomotor,  the  other  secretory.    The  for- 


o- 


Fig.  3. — The  nerve  supply  of  the  submaxillary  gland :  Li,  lingual  nerve  ; 
c.  t.,  chorda  tympani ;  g.  gland.  Wharton's  duct  is  ligated  and  it  will  be 
noticed  that  the  chorda  leaves  the  lingual  nerve,  just  before  this  crosses  the 
duct,   thus  forming  the  submaxillary  triangle.      (Claude   Bernard.) 


mer,  in  the  case  of  the  cerebral  nerves,  are  dilator  in  their  action, 
but  in  the  sympathetic  they  are  constrictor.  On  account  of  the 
association  of  secretory  and  vasodilator  fibers,  in  the  cerebral 
nerves,  stimulation  leads  to  the  secretion  of  large  quantities  of 
saliva,  the  amount  of  which,  as  well  as  its  percentage  of  organic 
and  inorganic  constituents,  varies  with  certain  limits  with  the 
strength  of  the  stimulus.  Although  secretory  activities  also  be- 
come excited  when  the  sympathetic  nerve  is   stimulated,    as   is 


42  HUMAN   PHYSIOLOGY. 

revealed  by  histological  examination  of  the  gland,  there  is  only 
a  slight  flow  of  saliva  from  the  duet  because  of  the  concomitant 
curtailment  of  the  blood  supply.  In  so  far  as  actual  secretion  of 
saliva  is  concerned,  the  net  result  of  stimulation  of  either  nerve 
is  therefore  dependent  upon  whether  dilatation  or  constriction 
of  the  blood  vessels  of  the  gland  occurs,  and  this  might  Jead  us 
to  conclude  that  the  secretion  is  secondary  to  changes  in  the 
blood  supply ;  in  other  words,  that  it  is  unnecessary  to  assume 
the  independent  existence  of  specific  secretory  nerve  impulses. 
That  such  secretory  fibers  do  exist,  however,  is  established  by 
many  facts.  Two  of  these  are:  (1)  The  vessels  still  dilate  but 
no  secretion  occurs  after  a  certain  amount  of  atropin  has  been 
allowed  to  act  on  the  gland.  This  alkaloid  paralyzes  the  secre- 
tory nerve  fibers,  but  has  no  action  on  those  concerned  in.  vaso- 
dilation. (2)  If  the  secretions  were  merely  the  result  of  in- 
creased blood  supply,  in  other  words,  were  a  filtrate  from  the 
blood,  the  pressure  in  the  duct  would  at  all  times  be  less  than 
that  in  the  blood  vessels;  but  this  is  not  the  case,  for  during  stim- 
ulation of  the  cerebral  nerves  the  duct  pressure  may  rise  far 
above  that  of  the  blood  vessels. 

But  it  must  never  be  lost  sight  of  that  although  both  kinds  of 
fibers  do  exist,  they  are  very  closely  associated  in  their  action. 

The  Reflex  Nervous  Control  of  Salivary  Secretion. — The 
structural  differences  between  the  parotid  and  submaxillary 
glands  suggest  that  their  functions  may  not  be  the  same;  that 
their  respective  secretions  must  be  required  for  different  pur- 
poses. To  put  this  supposition  to  the  test,  it  becomes  necessary 
to  adopt  some  means  by  which  the  conditions  calling  forth 
the  secretion  of  each  gland  may  be  separately  studied.  This  can 
be  accomplished  by  a  small  surgical  operation  in  which  the  ducts 
ar-e  transplanted  so  as  to  discharge  through  fistula  in  the  cheek, 
the  secretion  being  easily  collected,  by  allowing  it  to  flow  into  a 
funnel  which  is  tied  in  place. 

*  In  general,  two  distinct  types  of  stimuli  may  call  forth  secretion 
of  one  or  other  gland,  namely:  (1)  direct  stimulation  of  sensory 
nerve  endings  in  the  mouth,  and  (2)  psychological  stimuli  in- 
volving more  or  less  of  an  association  of  ideas. 


SALIVARY    SECRETION.  43 

Of  the  stimuli  which  cause  secretion  by  acting  on  sensory  nerve 
endings  in  the  mouth,  some  ijifluence  the  parotid,  others,  the  sub- 
maxillary gland,  and  ditiPerent  stimuli  produce  different  effects. 
Even  for  pure  mechanical  stimulation  of  the  buccal  mucosa,  a 
marked  degree  of  discrimination  is  shown;  thus,  smooth  clean 
pebbles  may  be  rolled  around  in  the  mouth  and  yet  cause  no 
saliva  to  be  secreted,  whereas  dry  sand  will  immediately  cause 
the  parotid  to  discharge  enormous  quantities  of  thin  watery 
juice.  Similarly  dry  bread  crumbs  invoke  copious  parotid  secre- 
tion, bread  itself  having  little  effect;  water,  ice,  etc.,  are  inert, 
but  if  they  contain  a  trace  of  acid  an  abundant  secretion  is  in- 
stantly poured  out.  It  is  plain  in  all  these  cases  that  the  pur- 
pose of  the  secretion  is  to  assist  in  the  removal  or  neutralization 
of  the  substance  which  is  present  in  the  mouth.  The  thick 
mucous  secretion  of  the  submaxillary  and  sublingual  glands 
seems  to  depend  more  on  the  chemical  nature  of  the  food  than  on 
its  mechanical  state,  boiled  potatoes,  hard  boiled  eggs,  meat,  etc., 
causing  the  secretion  of  a  thick  slimy  saliva,  which  by  coating 
the  food  assists  swallowing.  The  relish  for  the  food  seems  to  be 
of  little  account  in  influencing  the  secretion  of  saliva,  for  noxious 
substances,  or  those  that  are  acid,  or  very  salty,  call  forth  much 
more  secretion  than  do  savory  morsels.  Although  mere  mechani- 
cal stimulation  is  not  in  itself  an  adequate  stimulus,  yet  move- 
ment of  the  lower  jaw  is  (juite  effective,  as  for  example  in  chew- 
ing, or  when  the  mouth  is  kept  open,  as  by  ai  gag  in  a  dental 
operation. 

The  stimulus  does  not,  however,  require  to  be  applied  to  the 
buccal  mucosa  itself;  it  may  be  psychic  or  associational,  and 
liere  again  a  remarkable  discrimination  is  evident,  although  the 
response  is  not  so  predictable  as  when  the  stimulus  is  local. 
Thus,  when  dry  bread  or  sand  is  shown  to  a  dog  to  which  previ- 
ously these  substances  have  been  given  by  mouth,  salivation  fol- 
lows, but  this  is  not  the  case  when  moist  bread  or  pebbles  are 
offered.  Appetite  plays  an  important  part  in  this  psychic  reflex, 
for  when  dry  food  is  shown  to  a  fasting,  animal,  salivation  is 
marked,  but  may  cause  no  secretion  when  it  is  offered  to  a  well- 
fed  animal.    It  is  possible  in  this  case,  however,  that  there  may 


44  HUMAN   PHYSIOLOGY. 

be  inhibition  of  the  glandular  activities  on  account  of  the  pres- 
ence of  food  products  in  the  blood.  Perhaps  the  most  interesting 
fact  of  all  is  that  even  a  fasting  animal  will  after  a  time  fail  to 
salivate  if  he  be  repeatedly  shown  food  which  causes  a  secretion, 
but  which  he  is  not  permitted  to  get.  The  response  is  immedi- 
ately established  again,  however,  if  some  food,  or  indeed  some 
other  object,  be  placed  in  the  mouth.  A  hungry  animal  will  even 
salivate  when  he  hears  some  sound  which  by  previous  experience 
he  has  learned  to  associate  with  feeding  time.  The  psychic 
reflexes  are  evidently  dependent  upon  an  association  of  ideas  (a 
nervous  integration,  see  p.  242)  ;  they  are  conditioned  reflexes, 
and  are  therefore  the  result  of  a  certain  degree  of  education. 
They  are  easily  rendered  ineffective  by  confusing  the  usual  asso- 
ciations. 

General  Functions  of  Saliva. — These  observations  indicate 
that  a*  very  important  function  of  the  saliva  is  what  we  may  call 
a  mechanical  one,  namely,  either  to  flood  the  mouth  cavity  with 
fluid  and  so  to  wash  away  objectionable  objects  in  it,  or  to  lubri- 
cate the  food  with  mucin  and  so  facilitate  swallowing.  The  sol- 
vent action  of  saliva  is  also  important  for  the  act  of  tasting  (see 
p.  295).  Its  cJiemical  activities  in  many  animals  seem  to  be  lim- 
ited to  the  neutralizing  properties  of  the  alkali  which  is  present 
in  it,  but  in  man  and  the  herbivora  it  also  contains  a  certain 
amount  of  a  diastatic  enzyme,  ptyalin,  which  can  quickly  con- 
vert cooked  starches  into  dextrines  and  maltose.  Even  when  this 
action  is  most  pronounced,  however — for  it  varies  considerably 
in  different  individuals — it  cannot  proceed  to  any  extent  in  the 
mouth  cavity,  partly  on  account  of  the  short  time  food  remains 
here,  and  partly  because  many  starches,  as  in  biscuits,  are  taken 
more  or  less  in  a  raw  state.  In  some  animals,  such  as  the  dog, 
the  saliva  has  no  diastatic  action  whatever.  Although  there  can 
therefore  be  little  diastatic  digestion  in  the  mouth,  a  good  deal 
may  go  on  in  the  stomach,  for  the  saliva  that  is  swallowed  along 
with  the  food  does  not  become  destroyed  by  the  gastric  juice 
until  some  thirty  minutes  after  the  food  has  gained  the  stomach. 

Although  mastication  of  the  food  and  its  preparation  for 
swallowing  are  undoubtedly  the  main  functions  of  the  mouth  cav- 


SALIVARY    SECRETION.  45 

ity,  another  exists  which  is  of  very  great  importance  for  proper 
digestion;  this  is  the  stimulation  of  the  taste  nerve  endings, 
and,  for  foods  with  a  flavor,  of  those  of  the  olfactory  nerve  in  the 
posterior  nares.  Such  stimulation  not  only  gratifies  the  appetite, 
but  it  serves  as  the  adequate  stimulus  to  set  agoing  the  secretion 
of  the  gastric  juice.  Without  any  relish  for  food,  digestion  as 
a  whole  materially  suffers,  and  for  this  reason  unpalatable  food 
is  always  more  or  less  indigestible. 

Recent  investigations  point  to  another  function  of  the  saliva. 
Pepsin,  a  ferment  which  is  important  in  the  digestion  of  the 
proteins  and  which  is  found  in  the  juice  secreted  by  the  glands 
of  the  stomach,  is  readily  absorbed  by  starch  when  in  the  col- 
loidal state  as  it  is  generally  eaten.  •  In  this  condition  the  fer- 
ment is  not  free  to  act  upon  the  proteins  and  digestion  is  de- 
layed. If  the  saliva  be  allowed  to  partially  digest  the  starch 
into  sugar  before  the  food  reaches  the  stomach,  the  colloidal  state 
is  changed  by  the  action  of  the  ptyalin  of  the  saliva,  and  ab- 
sorption of  the  ferment  does  not  occur. 


CHAPTER  IV. 


DIGESTION  (Cont'd). 

The  Chemistry  of  Saliva  and  the  Relationship  of  Saliva  to 

Dental  Caries. 

A  knowledge  of  the  composition  and  chemical  properties  of 
saliva  is  of  great  importance  because  of  the  undoubted  etiologi- 
cal relationship  which  exists  between  this  secretion  and  dental 
caries.  Mixed  saliva  when  freshly  secreted  is  a  watery,  more  or 
less  opalescent  and  sticky  fluid,  often  containing  smalL  masses 
of  mucin,  but  on  standing  it  becomes  cloudy  because  of  precipi- 
tation of  calcium  carbonate.  Its  specific  gravity  is  1002-1006, 
and  it  contains  about  0.05  per  cent  of  solids.  The  saliva  from 
the  sublingual  and  submaxillary  glands  is  very  much  richer  in 
solids  than  that  from  the  parotid.  The  parotid  saliva  also  differs 
from  that  of  the  other  glands  in  containing  no  mucin,  although 
it  is  often  rich  in  ferment.  The  solid  constituents,  with  some  of 
their  properties,  are  as  follows: 

'  Glycoprotein  (mucin)  :  precipitated  by  acid. 
Other  proteins:  coagulated  by  heat. 
Organic.  .  .^  Ptyalin:  a  starch-splitting  enzyme. 

Potassium  sulphoeyanide :  gives  a  red  color  with 
ferric  chloride. 

Sodium  chloride :  )  give  a  precipitate  with  sil- 

Potassium  chloride :      j      ver  nitrate. 
Calcium  bicarbonate :  in  fresh  saliva. 
Calcium   carbonate :    precipitated   in   saliva    after 
Inorganic .  <j       standing. 


Calcium  and  magnesium 

phosphates : 
Sodium    and    potassium 

phosphates : 

Organic    Constituents, — Mucin    is    the    substance    to    which 
saliva  owes  its  stickiness.    Being  a  glycoprotein,  it  yields  reduc- 

46 


Have  an  important  re- 
lationship to  the  neu- 
tralizing properties 
of  saliva. 


CHEMISTRY    OP    SALIVA.  47 

ing  sugar  when  it  is  hyclrolyzed,  as  by  boiling  with  acid.  It  was 
at  one  time  suggested  that  sugar  might  sometimes  appear  in  the 
saliva,  as  a  result  of  bacterial  action  in  the  mouth,  and  be  respon- 
sible for  caries  of  the  teeth.  The  amount  thus  produced  is,  how- 
ever, so  very  small  in  comparison  with  the  ingested  carbohy- 
drates that  it  can  be  entirely  disregarded. 

Ptyalin. — This  belongs  to  the  class  of  diastatic  or  amylolytie 
enzymes,  converting  starch  into  sugar.  It  is  not  so  powerful  as 
the  similar  enzyme  in  pancreatic  juice  (see  p.  74),  for  it  has 
no  action  on  uncooked  starch,  wliich  the  latter  has.  It  acts  best 
in  neutral  reaction  and  in  the  presence  of  sodium  chloride,  but 
is  little  affected  by  a  small  degree  of  alkalinity.  On  the  other 
hand,  it  is  readily  destroyed  by  acids  and  by  higher  degrees  of 
alkalinit,y.  These  facts  are  of  importance  in  connection  with  the 
continuance  of  action  of  saliva  after  it  has  been  swallowed,  for 
although  the  food  remains  in  the  mouth  for  much  too  brief  a 
period  to  permit  of  more  than  a  trace  of  sugar  being  formed 
here,  yet,  after  the  stomach  is  reached,  ptyalin  may  continue  to 
act  for  about  half  an  hour.  The  ptyalin  content,  however,  varies 
very  considerably  in  different  individuals. 

Ptyalin  converts  starch  into  the  sugar  maltose,  so  called  be- 
cause it  is  also  formed  by  the  action  of  the  diastase  of  malt.  As 
intermediate  substances  are  formed  the  dextrins,  two  of  which 
are  distinguishable  on  account  of  their  behavior  towards  iodine ; 
one  of  these,  called  erythrodextrin,  gives  a  brown  color  with  this 
reagent,  while  the  other  gives  no  color  and  is  called  achroodex- 
trin. 

It  has  been  suggested  that  a  deficiency  of  ptyalin  may  pre- 
dispose to  caries  of  the  teeth  because,  under  such  circumstances, 
a  large  amount  of  dextrin  is  formed,  which  being  very  sticky  in 
character  adheres  to  the  teeth  and  becomes  a  suitable  nidus  for 
bacterial  growth. 

Potassium  Sulphocyanide  (sulphocyanate) . — This  salt  has 
the  formula  KCNS,  and  is  usually  present  in  human  saliva 
to  the  extent  of  about  0.01  per  cent.  It  is  produced  in  the  blood 
whenever  cyanides  or  organic  nitrites  make  their  appearance  in 
tlie  organism,  one  source  for  these  being  possibly  protein  meta- 


48  HUMAN   PHYSIOLOGY. 

holism  (p.  108).  It  is  excreted  from  the  "blood  into  the  urine  as 
well  as  the  saliva.  In  contrast  to  cyanides  it  is  non-poisonous,  so 
that  it  represents  the  innocuous  form  into  which  these  substances 
are  converted. 

The  chemical  test  used  for  its  detection  is  the  red  color  which 
it  gives  with  a  solution  of  ferHc  chloride  (FeClg).  Sometimes, 
■»  however,  the  reaction  is  not  very  definite,  in  which  ease  the 
method  of  Bunting  should  be  employed.  This  is  performed  as 
follows :  Slowly  evaporate  5  c.  c.  of  saliva  in  a  watch  glass  and 
while  stirring  with  a  glass  rod  add  a  few  drops  of  a  26  per  cent 
solution  of  FeClg.  Pour  about  5  c.  c.  of  a  mixture  of  5  parts 
amyl  alcohol  and  2  parts  ether  over  the  residue,  and  after  stir- 
ring decant  into  a  test  tube.  If  sulphocyanide  is  present,  the 
alcohol-ether  will  become  red.  Benzoate  and  aceto  acetic  acid 
may  give  a  similar  reaction,  but  most  of  the  other  substances 
which  might  interfere  with  the  test,  as  when  it  is  done  by  merely 
adding  FeClg  to  saliva,  are  eliminated  by  Bunting's  method. 

All  this  care  and  interest  in  the  testing  for  KCNS  has  arisen 
because  of  the  supposition  that  the  amount  of  this  substance  in 
saliva  might  have  some  relationship  to  caries  of  the  teeth.  It 
was  suggested  that  it  might  confer  on  the  saliva  somewhat  of  an 
.  antiseptic  action  and  thus  destroy  the  bacteria  that  are  the  cause 
of  caries.  Careful  work  by  Bunting,  by  Gies  and  others  has, 
however,  shown  that  this  hypothesis  is  untenable. 

Inorganic  Constituents. — Two  important  questions  arise  in 
connection  with  these,  viz:  (1)  their  relationship  to  the  reaction 
of  the  saliva;  (2)  the  conditions  which  control  the  precipitation 
of  calcium  carbonate  and  phosphate  and  the  deposition  of  the 
precipitate  on  the  teeth  in  the  form  of  tartar. 

The  Reaction  of  the  Saliva. — Tested  with  litmus  paper,  sa- 
liva is  more  or  less  alkaline  and  it  is  distinctly  so  towards  lac- 
moid  and  Congo  red,  but  it  is  acid  when  tested  with  phenolph- 
thalein.  It  is  thus  said  to  be  amphoteric,  like  blood  and  urine 
Difficulty  in  deciding  as  to  the  reaction  of  saliva  is  partly  due 
to  the  fact  that  it  changes  on  standing  because  carbon-dioxide 
(CO2)  is  dissipated,  thus  making  it  more  alkaline.  To  succeed 
in  determining  the  reaction  of  saliva,  we  must  therefore  under- 


CHEMISTRY    OF    SALIVA.  49 

stand  to  what  its  amphoteric  behavior  is  due  and  we  must  con- 
stantly bear  in  mind  that  the  real  reaction  of  a  fluid  is  the 
ratio  between  free  H-  and  OH-ions  (see  p.  30).  By  analysis 
saliva  has  been  found  to  contain  phosphates  and  carbonates,  both 
of  which  are  capable  of  existing  either  as  acid  or  alkaline  salts, 
that  is  to  say,  as  NaH2P04  and  NaHCOs  (acid  salts)  or  NaaHPO^ 
and  NaoCOg  (alkaline  salts).  Since  the  reaction  given  by  solu- 
tions which  contain  such  mixtures  of  acid  and  alkaline  salts 
depends,  first,  on  the  relative  proportions  of  these  salts  and, 
secondly,  on  the  exact  indicator  employed  to  test  the  reaction 
(see  p.  31),  it  is  plain  that  the  reaction  of  the  saliva  as  ordi-^ 
narily  tested  must  be  very  haphazard. 

To  determine  the  H-ion  concentration  of  saliva,  some  of  this 
fluid  is  diluted  about  ten  times  with  distilled  water,  which  has 
been  boiled  and  cooled  so  as  to  free  it  of  carbon  dioxide,  and 
0.5  c.  c.  of  paranitrophenol  solution  is  added.  The  resulting  tint 
is  then  compared  with  that  obtained  by  adding  0.5  c.  c.  of  the 
same  indicator  to  each  of  a  series  of  test  tubes  containing  vary- 
ing proportions  of  acid  and  alkaline  phosphate  solutions  (^/jg 
normal).  The  H-ion  concentration  of  the  saliva  is  equal  to  that 
of  the  phosphate  solution  which  gives  the  same  test. 

The  Method  of  Measuring  the  Neutralizing  Power  op  Sa- 
liva.— Interesting  though  H-ion  results  may  be,  they  do  not  ap- 
pear to  be  of  any  practical  value  in  connection  with  the  relation- 
ship between  the  saliva  and  caries  of  the  teeth.  To  study  this 
question  it  has  been  found  to  be  of  more  value  to  determine  the 
neutralizing  power  of  saliva ;  that  is,  to  find  out  how  much  stand- 
ard acid  or  alkali  we  must  add  to  a  measured  quantity  of  saliva 
in  order  to  get  a  change  with  one  or  more  of  the  above  indicators. 
In  doing  this,  however,  we  are  immediately  struck  with  the  fact 
that  the  reaction  does  not  change  in  proportion  to  the  amount 
of  acid  or  alkali  added,  but  that  the  saliva  under  such  conditions 
[)ossesses  the  property  of  changing  very  slowly  in  reaction.  This 
same  property  also  exists  in  the  blood,  and  it  depends  on  a  series 
of  changes  which  the  phosphates  and  carbonates  can  undergo, 
when  acids  or  alkalies  are  added  to  solutions  containing  them. 


50  HUMAN   PHYSIOLOGY. 

without  causing  any  considerable  amount  of  free  H-  or  OH-ion  to 
be  set  free.  '  This  has  been  called  the  "buffer  action"  of  such 
salts.  It  endows  the  saliva  with  the  power  of  locking  away  con- 
siderable quantities  of  acid  or  alkali. 

In  actually  measuring  the  neutralizing  power  of  saliva,  it  is 
best  first  of  all  to  bring  the  saliva  to  some  readily  detectable 
H-ion  concentration,  on  the  acid  side  of  neutrality,  and  then  to 
find  out  how  much  alkali  is  required  to  bring  it  to  another  defi- 
nite H-ion  concentration,  on  the  alkaline  side. 

The  methods  for  applying  the  above  principles  are  as  follows : 

10  c.c.  saliva  is  diluted  in  an  evaporating  dish  with  20  c.c. 
water  which  has  been  boiled  to  expel  COo  and  then  cooled  to 
20°C.  About  eight  drops  of  an  aqueous  solution  of  paranitro- 
phenol  is  then  added  and  N/200  HCl  run  in  from  a  burette,  with 
constant  stirring  until  the  yellow  color  due  to  the  indicator  just 
disappears.  The  amount  of  N/200  HCl  is  noted.  N/200  NH^HO 
is  then  added  till  the  yellow  color  just  returns.^  The  difference 
between  the  two  readings  gives  the  alkalinity  in  terms  of  c.c.  of 
N/200  HCl. 

The- acidity  may  be  directly  measured  by  adding  four  drops 
of  an  alcoholic  solution  of  phenolphthalein  to  another  10  c.c. 
sample  of  saliva  and  running  in  N/200  NaOH  until  a  definite 
pink  color  results. 

Addition  of  the  acidity  and  alkalinity  results  gives  the  total 
neutralizing  power  of  the  saliva,  or  in  other  words  the  power  of 
maintaining  neutrality.  This  is  a  much  more  constant  property 
of  saliva  than  the  acidity  or  alkalinity  alone,  and  it  has  conse- 
quently been  used,  in  the  most  recent  work  of  Marshall,^  for  the 
purpose  of  ascertaining  whether  the  susceptibility  to  dental  ca- 
ries bears  any  relationship  to  the  reaction  of  the  saliva.  It  was 
found  that  it  does  not.    On  the  other  hand,  this  author  has  shown 


iThe  reason  for  titrating  back  witli  N/200  NH4HO  till  a  yellow  color  again 
reappears,  when  measuring  the  alkalinity,  is  to  increase  the  accuracy  of  the 
titration,  it  being  often  difficult  to  decide  the  point  at  which  the  color  disap- 
pears when  N/200  acid  is  added,  but  easy  to  decide  when  it  reappears  when 
N/200  NH4HO  is  used.  NaOH  is  employed  in  the  acidity  titration  because 
phenolphthalein  cannot  be  used  with  ammonia. 

2Cf.  J.  A.  Marshall,  Amer.   Jour,  of  Physiology,   1915,   XXXVI,  p.   260. 


CHEMISTRY    OF    SALIVA. 


51 


that  the  neutralizing  power  of  saliva,  collected  without  any  ef- 
fort or  artificial  stimulation  of  the  mouth  (resting  saliva),  is 
very  distinctly  less  than  that  of  saliva  collected  whilst  chewing 
on  a  piece  of  paraffin  (activated  saliva),  and  that  this  difference 
becomes  nery  much  less  in  those  with  carious  teeth.  Marshall 
has  suggested  that  we  should  express  the  ratio  of  the  neutraliz- 
ing power  of  resting  saliva  to  that  of  activated  saliva  as  a  per- 
centage ratio,  which  he  calls  the  salivary  factor.  In  persons  im- 
mune from  caries  this  factor  amounted  to  43-80;  in  those  with 
caries  it  varied  from  80-132.  The  following  examples  will  illus- 
trate these  points : 


NORMAL  BESTING   SALIVA 


ACTIVATED   SALIVA 


Case 

c.  c. 

N/200 
HCl 

c.  c. 

N/200 
NaOH 

Neutral- 
izing 
Power 

c.  c. 

N/200 
HCl 

c.  c. 
N/200 
NaOH 

Neutral- 
izing 
Power 

Salivary 
Factor 

No 
caries 

22.22 

7.60 

29.82 

57.55 

0.90 

58.45 

51 

Carious 

18.50 

7.00 

25.50 

22.80 

2.13 

24.93 

102 

If  these  interesting  observations  should  prove  to  be  confirmed 
by  other  observers,  a  comparatively  simple  method  will  be  avail- 
able for  solving  what  has  hitherto  been  a  most  puzzling  ques- 
tion. Several  observers,  particularly  Bunting  and  Price,  have 
employed  the  very  delicate  physico-chemical  methods  of  the  con- 
centration cell  (p.  31)  and  electrical  conductivity  in  their  at- 
tempts to  solve  it. 

Before  leaving  the  subject  of  the  relationship  between  the 
character  of  the  saliva  and  the  occurrence  of  dental  caries,  it 
may  be  well  to  point  out  that  other  factors  besides  the  neutral- 
izing power  of  the  saliva  must  be  taken  into  consideration,  name- 
ly, its  amount  and  the  presence  of  phosphates.  A  large  and  free 
flow  of  saliva,  besides  mechanically  cleansing  the  teeth,  will  offer 
more  neutralizing  fluid.  An  excess  of  phosphates,  on  the  other 
hand,  will  encourage  fermentation  of  any  carbohydrate  which 
may  be  adherent  to  the  teeth  and,  by  forming  acids,  thus  tend  to 
erode  the  teeth  and  predispose  to  caries. 


52  HUMAN  PHYSIOLOGY. 

Tartar  Formation  and  Salivary  Calculi. — Under  certain  con- 
ditions a  precipitate,  varying  in  color  from  pale  yellow  to  almost 
black,  collects  on  tlie  teeth,  particularly  on  the  lower  incisors 
and  molars.  This  precipitate  is  called  tartar,  and  it  may  be 
either  hard  (as  on  the  incisors)  or  soft  (as  on  the  molars).  Its 
chemical  composition  varies  considerably,  but  may  be  given  as 
follows : 

I  II 

Water   and   organic   matter 32.24  per  cent        31.48  per  cent 

Magnesium  phosphate 0.98  per  cent  4.91  per  cent 

Calcium  phospliate 63.08  per  cent        72.73  per  cent 

Calcium  carbonate  3.7    percent         ■ 

(Talbot) 

The  organic  matter  consists  of  epithelial  scales,  other  extran- 
eous matter  and  leptothrix  chains.  The  place  and  manner  of 
deposition  shows  clearly  that  the  tartar  is  largely  derived  from 
the  saliva,  the  chemical  explanation  of  the  precipitation  being 
probably  as  follows :  Saliva,  as  it  is  produced  in  the  gland,  con- 
tains calcium  bicarbonate,  which  is  soluble  in  water,  and  is  pre- 
vented from  changing  into  the  insoluble  carbonate  by  the  pres- 
ence of  free  carbon  dioxide  in  solution.  When  the  saliva  is  dis- 
charged into  the  mouth  some  of  the  carbon  dioxide  escapes  from 
it  so  that  the  bicarbonate  changes  to  carbonate  and  becomes  pre- 
cipitated. The  precipitate  carries  down  with  it  phosphates  as  well 
as  any  organic  debris  or  mico-organisms  that  may  be  present. 

The  precipitation  of  calcium  carbonate  may  even  take  place 
in  the  salivary  ducts  ("Wharton's),  thus  forming  salivary  calculi, 
which  may  reach  the  size  of  a  pea  or  larger.  Such  calculi  may 
contain  as  much  as  3.8  per  cent  of  organic  matter,  the  remainder 
being  largely  calcium  carbonate.  The  following  table  gives  the 
composition  of  three  such  calculi :  * 

I  II                         III 

Calcium   carbonate    81.2  per  cent  79.4  per  cent        80.7  per  cent 

Calcium   phosphate   4.1  per  cent  5.0  per  cent          4.2  per  cent 

Magnesium  phosphate  ...     i  present 
Organic  matter  and  other 

soluble  solids 13.3  per  cent  13.3  per  cent        13.4  per  cent 

Water 1.3  per  cent  2.3  per  cent          1.7  per  cent 

(Talbot) 


CHAPTER  V. 

DIGESTION  (Cont'd). 

Mastication :   Deglutition :   Vomiting. 

Mastication. — By  the  movements  of  the  lower  jaw  on  the 
upper,  the  two  rows  of  teeth  come  together  so  as  to  serve  for  bit- 
ing or  crushing  the  food.  The  resulting  comminution  of  the  food 
forms  the  first  step  in  digestion.  The  manner  of  occlusion  of  the 
cusps  of  the  teeth  in  the  performance  of  this  act  is  not  a  problem 
of  Physiology,  but  rather  of  Anatomy  and  Orthodontics;  never- 
theless, the  other  factors  which  contribute  to  the  efficiency  of  the 
process  and  the  condition  into  which  the  food  is  brought  by  it 
are  subjects  to  which  we  must  devote  some  attention.  The  up 
and  down  motion  of  the  lower  jaw  results  in  biting  by  the  in- 
cisors, and  after  the  mouthful  has  been  taken,  the  side  to  side 
movements  enable  the  grinding  teeth  to  crush  and  break  it  up 
into  fragments  of  the  proper  size  for  swallowing.  The  most  suit- 
able size  of  the  mouthful  is  about  five  cubic  centimeters,  but  this 
varies  greatly  with  habit.  After  mastication,  the  mass  weighs 
from  3.2  to  6.5  gram,  about  one-fourth  of  this  weight  being 
due  to  saliva.  The  food  is  now  a  semi-fluid  mush  containing  par- 
ticles which  are  usually  less  than  2  millimeters  in  diameter. 
Some,  however,  may  measure  7  and  even  12  millimeters. 

Determination  of  the  proper  degree  of  fineness  of  the  food  is  a 
function  of  the  tongue,  gums  and  cheeks,  for  which  purpose  the 
mucous  membrane  covering  them  is  supplied  with  very  sensitive 
touch  nerve  endings  (see  p.  244).  The  sensitiveness  of  the 
tongue,  etc.,  in  this  regard  explains  why  an  object  which  can 
scarcely  be  felt  by  the  fingers  seems  to  be  quite  large  in  the 
mouth.  If  some  i)articles  of  food  that  are  too  large  for  swallow- 
ing happen  to  be  carried  backward  in  the  mouth,  the  tongue  re- 
turns them  for  further  mastication. 

The  saliva  assi.sts  in  mastication  in  several  ways:  (1)  by  dis- 

53 


54  "*  HUMAN   PHYSIOLOGY. 

solving  some  of  the  food  constituents;  (2)  by  partially  digesting 
some  of  the  starch;  (3)  by  softening  the  mass  of  food  so  that  it 
is  more  readily  crushed ;  (4)  by  covering  the  bolus  with  mucus 
so  as  to  make  it  more  readily  transferable  from  place  to  place. 
The  secretion  of  saliva  is  therefore  stimulated  by  the  chewing 
movements,  and  its  composition  varies  according  to  the  nature 
of  the  food  (p.  43).  In  some  animals,  such  as  the  cat  and  dog, 
there  is  no  mastication,  the  food  being  merely  coated  with  sa- 
liva and  then  swallowed.  In  man  the  ability  thus  to  bolt  the 
food  can  readily  be  acquired,  not  however  without  some  detri- 
ment to  the  efficiency  of  digestion  as  a  whole.  Soft  starchy  food 
is  little  chewed,  the  length  of  time  required  for  the  mastication 
of  other  foods  depending  mainly  on  their  nature,  but  also  to  a 
certain  degree  on  the  appetite  and  the  size  of  the  mouthful. 
The  crushing  force  of  the  molars,  as  measured  by  a  dyna- 
mometer, can  be  made  to  rise  as  high  as  270  pounds,  but  this  is 
far  in  excess  of  the  force  required  to  crush  the  ordinary  food 
stuffs.  Thus  cooked  meats  have  a  crushing  point  which  varies 
between  15  and  80  pounds  on  direct  thrust,  but  is  considerably 
less  when  there  is  a  side  to  side  movement,  as  there  is  in  chewing. 
Candies  have  a  crushing  point  of  30  to  110  pounds,  and  nuts 

55  to  170  pounds.  Admixture  of  the  food  with  saliva  greatly 
lowers  the  crushing  point,  especially  in  the  case  of  such  foods  as 
soft  bread.  Without  such  admixture  this  hardens  into  a  solid 
mass  when  it  is  crushed,  whereas  it  readily  breaks  up  into  small 
particles  in  the  presence  of  saliva. 

It  cannot  be  too  strongly  insisted  upon  that  the  act  of  masti- 
cation is  of  far  more  importance  than  merely  to  break  up  and 
prepare  the  food  for  swallowing.  It  causes  the  food  to  be  moved 
about  in  the  mouth  so  as  to  develop  its  full  effect  on  the  taste 
buds;  the  crushing  also  releases  odors  which  stimulate  the  ol- 
factory epithelium.  On  these  stimuli  depend  the  satisfaction 
and  pleasure  of  eating,  which  in  turn  initiate  the  process  of  gas- 
tric digestion  (see  p.  60).,  Thus  it  has  been  observed  in  chil- 
dren with  gastric  fistul^e  that  the  chewing  of  agreeable  food 
caused  the  gastric  juice  to  be  actively  secreted,  which,  however, 
was  not  the  case  when  tasteless  material  was  chewed. 


DEGLUTITION. 


55 


The  benefit  to  digestion  as  a  whole  of  a  large  secretion  of  sa- 
liva, brought  about  by  persistent  chewing,  has  been  assumed  by 
some  to  be  much  greater  than  it  really  is,  and  there  has  existed, 
and  indeed  may  still  exist,  a  school  of  faddists  who,  by  deliber- 
ately chewing  far  beyond  the  necessary  time,  imagine  themselves 
to  thrive  better  on  less  food  than  those  who  occupy  their  time 
with  other  more  profitable  pursuits. 

Deg'lutition  or  Swallowing. — After  being  masticated  the  food 
is  rolled  up  by  tlie  tongue,  acting  against  the  palate,  into  a  bolus, 
and  this,  after  being  lubricated  by  saliva,  is  moved,  by  elevation 


Fig.  4. — The  changes  which  take  place  in  the  position  of  the  root  of  the 
tongue,  the  soft  palate,  the  epiglottis  and  the  larynx  during  the  second  stage 
of  swallowing.  The  thick  dotted  line  indicates  the  position  during  swal- 
lowing. 

of  the  front  of  the  tongue,  towards  the  back  of  the  mouth.  This 
constitutes  the  first  stage  of  swallowing,  and  is,  so  far,  a  volun- 
tary act.  About  this  time  a  slight  inspiratory  contraction  of  the 
diaphragm  occurs — the  so-called  respiration  of  swallowing — and 
the  mylohyoid  muscle  of  the  floor  of  the  mouth  quickly  con- 
tracts with  the  consequence  that  the  bolus  passes  between  the 


56  HUMAN   PHYSIOLOGY. 

pillars  of  the  fauces.  This  marks  the  beginning  of  the  second 
stage,  the  first  event  of  which  is  that  the  bolus,  by  stimulating 
sensory  nerve  endings,  acts  on  nerve  centers  situated  in  the  me- 
dulla oblongata  so  as  to  cause  a  coordinated  series  of  movements 
of  the  muscles  of  the  pharynx  and  larynx  and  an  inhibition  for 
a  moment  of  the  respiratory  center  (p.  219).  The  movements 
alter  the  shape  of  the  pharynx  and  of  the  various  openings  into 
it  in  such  a  manner  as  to  compel  the  bolus  of  food  to  pass  into 
the  oesophagus:  (see  Fig.  4)  thus,  (1)  the  soft  palate  becomes 
elevated  and  the  posterior  wall  of  the  pharynx  bulges  forward 
so  as  to  shut  off  the  posterior  nares,  (2)  the  posterior  pillars  of 
the  fauces  approximate  so  as  to  shut  off  the  mouth  cavity,  and 
(3)  in  about  a  tenth  of  a  second  after  the  mylohyoid  has  con- 
tracted, the  larynx  is  pulled  upwards  and  forwards  under  the 
root  of  the  tongue,  which  by  being  drawn  backwards  becomes 
banked  up  over  the  laryngeal  opening.  This  pulling  up  of  the 
larynx  brings  the  opening  into  it  near  to  the  lower  half  of  the 
dorsal  side  of  the  epiglottis,  but  the  upper  half  of  this  structure 
projects  beyond  and  serves  as  a  ledge  to  guide  the  bolus  safely 
past  this  critical  part  of  its  course.  (4)  To  further  safeguard 
any  entry  of  food  into  the  air  passages,  the  laryngeal  opening  is 
narrowed  by  approximation  of  the  true  and  false  vocal  cords. 

The  force  which  propels  the  bolus,  so  far,  is  mainly  the  con- 
traction of  the  mylohyoid,  assisted  by  the  movements  of  the  root 
of  the  tongue.  When  it  has  reached  the  lower  end  of  the 
pharynx,  however',  the  bolus  readily  falls  into  the  oesophagus, 
which  has  become  dilated  on  account  of  a  reflex  inhibition  of  the 
constrictor  muscles  of  its  upper  end.  This  so-called  second  stage 
of  swallowing  is  therefore  a  complex  coordinated  movement  ini- 
tiated by  afferent  stimuli  and  involving  reciprocal  action  of 
various  groups  of  muscles :  inhibition  of  the  respiratory  muscles 
and  of  those  that  constrict  the  oesophagus,  and  stimulation  of 
those  that  elevate  the  palate,  the  root  of  the  tongue  and  the 
larynx.    It  is  purely  an  involuntary  process. 

The  third  stage  of  deglutition  consists  in  the  passage  of  the 
swallowed  food  along  the  oesophagus.  The  way  in  which  this  is 
done  depends  very  much  on  the  physical  consistence  of  the  food. 


DEGLUTITION.  57 

A  solid  dolus,  that  more  or  less  fills  the  oesophagus,  excites  a 
typical  peristaltic  wave,  which  is  characterized  by  a  dilatation 
of  the  oesophagus  immediately  in  front  of,  and  a  constriction 
over  and  behind  the  bolus.  This  wave  travels  down  the  oesopha- 
gus at  such  a  rate  that  it  reaches  the  cardiac  sphincter  in  about 
five  or  six  seconds.  On  arriving  here  the  cardiac  sphincter, 
ordinarily  contracted,  relaxes  for  a  moment  so  that  the  bolus 
passes  into  the  stomach.  The  peristaltic  wave  travels  much  more 
rapidly  in  the  upper  portion  of  the  oesophagus  than  lower  down 
because  of  differences  in  the  nature  of  the  muscular  coat,  this 
being  of  the  striated  variety  above,  and  of  the  non-striated,  be- 
low. The  purpose  of  more  rapid  movement  in  the  upper  portion 
is  no  doubt  that  the  bolus  may  be  hurried  past  the  regions, 
where,  by  distending  the  oesophagus,  it  might  interfere  with  the 
function  of  neighboring  structures,  such  as  the  heart.  The  peris- 
taltic wave  of  the  oesophagus,  unlike  that  of  the  intestines  (see 
p.  79),  is  transmitted  by  nerves,  namely,  by  the  oesophageal 
branches  of  the  vagus.  If  these  be  severed,  but  the  muscular 
coat  left  intact,  the  oesophagus  becomes  dilated  above  the  level 
of  the  section  and  contracted  below,  and  no  peristaltic  wave  can 
pass  along  it ;  on  the  other  hand,  the  muscular  coat  may  be  sev- 
ered (by  crushing,  etc.)  but  the  peristaltic  wave  will  jump  the 
breach,  provided  no  damage  has  been  done  to  the  nerves. 

The  propagation  of  the  wave  by  nerves  indicates  that  the  sec- 
ond and  third  stages  of  deglutition  must  be  rehearsed,  as  it  were, 
in  the  nerve  centers  from  which  arise  the  fibers  to  the  pharynx 
and  the  different  levels  of  the  oesophagus.  The  afferent  stimuli 
wJiich  initiate  this  process  arise,  not  as  might  be  expected,  in 
the  a'sophagus  itself,  but  in  the  pharynx,  and  they  are  carried 
to  the  brain  by  the  fifth,  superior  laryngeal  and  vagus  nerves; 
thus,  a  foreign  body  placed  directly  in  the  oesophagus  does  not 
begin  to  move  until  the  pharynx  is  stimulated,  as  by  touching 
it.  The  afferent  fibers  in  the  glossopharyngeal  nerve  exercise 
a  i)0werful  inhibitory  influence  on  the  deglutition  center  as  well 
as  on  that  of  respiration.  Thus,  if  swallowing  movements  be 
excited  by  stimulating  the  central  end  of  the  superior  laryngeal 
nerve,    they    can    be    instantly     inhibited   by   stimulating   the 


58  HUMAN   PHYSIOLOGY, 

glossopharyngeal,  and  the  respiratory  movements  stop  in  what- 
ever position  they  may  have  been  at  the  time. 

This  inhibition  of  the  oesophagus  is  indeed  a  most  important 
part  of  the  process  when  liquid  or  semi-liquid  food  is  swallowed 
By  the  contraction  of  the  mylohyoid  muscle,  fluids  are  quickly 
shot  down  the  distended  oesophagus,  at  the  lower  end  of  which, 
on  account  of  the  cardiac  sphincter  being  closed,  they  accumulate 
until  the  arrival  of  the  peristaltic  wave  which  has  meanwhile 
been  set  up  by  stimulation  of  the  pharynx.  If  the  swallowing  is 
immediately  repeated,  as  is  usually  the  case  in  drinking,  the 
cesophagus  remains  dilated  because  peristalsis  is  inhibited,  and 
the  fluid  lies  outside  the  cardiac  orifice  until  the  last  mouthful 
has  been  taken. 

These  facts  have  been  revealed  by  listening  with  a  stethescope 
to  the  sounds  produced  by  swallowing,  and  by  observing  with  an 
X-ray  lamp  the  shadows  produced  along  the  course  of  the 
oesophagus  when  food,  impregnated  with  bismuth  subnitrate,  is 
taken.  When  a  solid  bolus  is  swallowed,  one  sound  is  usually 
heard,  but  with  liquid  food  there  are  two,  one  at  the  upper  end, 
due  to  the  rush  of  the  fluid  and  air,  and  the  other  occurring  four 
or  six  seconds  later  at  the  lower  end  (heard  over  the  epigas- 
trium), and  due  to  the  arrival  of  the  peristaltic  wave  with  the 
accompanying  opening  of  the  cardiac  sphincter  and  the  escape 
of  the  fluid  and  air  into  the  stomach.  Sometimes,  as  when  the  per- 
son is  in  the  horizontal"' position,  this  second  sound  may  be  broken 
up  into  several,  indicating  that,  unassisted  by  gravity,  the  fluid 
does  not  so  readily  pass  through  the  sphincter.  The  X-ray  shad- 
ows yield  results  in  conformity  with  the  above.  After  swallow- 
ing milk  and  bismuth,  for  example,  the  shadow  falls  quickly  to 
the  lower  end  of  the  cesophagus  and  then  slowly  into  the  stomach. 
When  the  passage  of  a  solid  bolus  is  watched  by  the  X-ray 
method,  its  rate  of  descent  will  be  found  to  depend  on  whether 
or  not  it  is  well  lubricated  with  saliva ;  if  not  so,  it  may  take  as 
long  as  fifteen  minutes  to  reach  the  stomach ;  if  moist,  but  from 
eight  to  eighteen  seconds. 

The  Act  of  Vomiting. — This  is  usually  preceded  by  a  feeling 
of  sickness  or  nausea  and  is  initiated  by  a  very  active  secretion 


VOMITING.  ■  59 

of  saliva.  The  saliva,  mixed  with  air,  accumulates  to  a  consider- 
able extent  at  the  lower  end  of  the  oesophagus  and  thus 
distends  it.  A  forced  inspiration  is  now  made,  during  the 
first  stage  of  which  the  glottis  is  open  so  that  the  air  enters 
the  lungs,  but  later  the  glottis  closes  so  that  the  in- 
spired air  is  sucked  into  the  oesophagus,  which,  already 
somewhat  distended  by  saliva,  now  becomes  markedly  so.  The 
abdominal  muscles  then  contract  so  as  to  compress  the  stomach 
against  the  diaphragm  and,  simultaneously,  the  cardiac  sphincter 
relaxes,  the  head  is  held  forward  and  the  contents  of  the  stomach 
are  ejected  through  the  previously  distended  oesophagus.  The 
compression  of  the  stomach  by  the  contracting  abdominal  mus- 
cles is  assisted  by  an  actual  contraction  of  the  stomach  itself, 
as  has  been  clearly  demonstrated  by  the  X-ray  method.  (See  p. 
58.)  After  the  contents  of  the  stomach  itself  have  been  evac- 
uated, the  pyloric  sphincter  may  also  relax  and  thus  permit  the 
contents  (bile,  etc.)  of  the  duodenum  to  be  vomited. 

The  act  of  vomiting  is  controlled  by  a  center  located  in  the 
medulla,  and  the  afferent  fibers  to  this  center  may  come  from 
many  different  regions  of  the  body.  Perhaps  the  most  potent 
of  them  come  from  the  sensory  nerve  endings  of  the  fauces  and 
pharynx.  This  explains  the  tendency  to  vomit  when  the  mucosa 
of  this  region  is  mechanically  stimulated.  Other  afferent  im- 
pulses come  from  the  mucosa  of  the  stomach  itself,  and  these  are 
stimulated  by  swallowing  certain  drugs  called  emetics,  import- 
ant among  which  are  strong  salt  solution,  mustard  water,  zinc 
sulphate,  etc.  When  some  poisonous  substance  has  been  swal- 
lowed, the  immediate  treatment  is  to  give  one  of  these  emetics 
and  thus  cause  the  poison  to  be  vomited.  Certain  other  emetics, 
particularly  tartar  emetic  and  apomorphine,  act  on  the  vomiting 
center  itself,  and  can  therefore  act  when  given  subcutaneously. 
Afferent  vomiting  impulses  also  arise  from  the  abdominal  vis- 
cera, thus  explaining  the  vomiting  which  occurs  in  strangulated 
hernia,  and  in  other  irritative  lesions  involving  this  region. 


CHAPTER  VI. 

DIGESTION  (Cont'd). 

Digestion  in  the  Stomach. 

The  Secretion  of  Gastric  Juice. — After  passing  the  cardiac 
sphincter,  the  food  collects  in  the  fundus  of  the  stomach.  When 
it  is  solid  in  consistency  it  becomes  disposed  in  definite  layers, 
the  first  swallowed  near  the  mucosa,  ^he  last  swallowed  in  the 
center.  When,  as  is  usual  in  man,  the  food  is  more  or  less  fluid, 
it  collects  in  the  most  dependent  part  of  the  body  of  the  stomach 
and  the  layer  formation  is  less  evident  (see  Fig.  5).  Within  a 
few  minutes  of  the  entry  of  the  first  portion  of  food,  the  glands 
of  the  gastric  mucosa  begin  to  secrete  their  digestive  juices.  The 
immediate  exciting  cause  of  this  secretion  is  not  the  contact  of 
food  with  the  mucosa — although  this  acts  later — but  is  a  ner- 
vous stimulus  transmitted  to  the  stomach  through  the  vagus 
nerve^  and  coming  from  a  nerve  center  situated  in  the  medulla. 

The  activities  of  this  gastric  center  are  called  into  operation  hy 
afferent  impulses  in  the  nerves  that  terminate  in  the  taste  huds 
and  olfactory  epithelium.  The  process  of  gastric  secretion  is 
therefore  initiated  in  the  mouth,  and  the  stimulus  that  is  re- 
sponsible for  it  is  the  good  taste  and  the  flavor  of  the  food.  Just 
as  in  the  case  of  the  salivary  glands,  the  food,  in  order  to  excite 
the  secretion,  need  not  actually  enter  the  mouth,  for  a  psychologi- 
cal stimulus  may  also  act  on  the  gastric  center.  Thus,  the  sight 
or  smell  of  savory  food,  or  even  the  hearing  of  some  sound  that  is 
known  by  experience  to  be  associated  with  the  gratification  of 
the  appetite  can  call  it  forth.  These  important  facts  were  first 
of  all  revealed  by  observations  through  a  gastric  fistula  (arti- 
ficial opening)  in  the  case  of  a  boy  who,  because  of  stricture  of 
the  oesophagus,  was  unable  to  take  food  by  the  mouth.     This 


lAfter  the  vagi  ai-e  cut,   this  secretion  of  gastric  juice  does   not  occur. 

60 


DIGESTION   IN   THE   STOMACH. 


61 


boy  had  to  be  fed  tlirough  the  gastric  fistula,  but  it  was  noticed 
that  when  he  was  allowed  to  chew  food  for  which  he  had  a  relish 
and  then  spit  it  out,  gastric  secretion  occurred.  This  observa- 
tion suggested  to  Pavlov  the  establishment  of  analogous  condi- 
tions in  dogs,  with  the  modification  that,  besides  the  fistula  in 
the  stomach,  another  was  made  in  the  oesophagus.  The  animal 
could  therefore  swallow  interminably  without  ever  becoming 
satisfied,  because  the  food  escaped  by  the  oesophageal  fistula. 


Fig.  5. — Diagrams  of  outline  and  position  of  stomach  as  indicated  by  skia- 
grams taken  on  man  in  the  erect  position  at  intervals  after  swallowing  food 
impregnated  with  bismuth  subnitrate.  A,  moderately  full ;  B,  practically 
empty.  The  clear  space  at  the  upper  end  of  the  stomach  is  due  to  gas,  and 
it  will  be  noticed  that  this  "stomach  bladder"  lies  close  to  the  heart.  (T. 
Wingate  Todd.) 


Nevertheless  the  gastric  juice  flowed  abundantly,  provided  this 
"sham  feeding"  was  with  appetizing  food.  Stones,  bread,  acid  or 
irritating  substances,  although  they  might  cause  much  saliva  to 
Ijc  secreted  and  .swallowed  (see  p.  43),  had  no  influence  whatso- 
ever on  the  flow  of  gastric  juice.  The  only  adequate  stimulus 
was  gratification  of  the  appetite. 


62 


HUMAN   PHYSIOLOGY. 


In  passing,  it  may  be  well  to  call  attention  to  the  practical 
importance  of  these  observations  in  connection  with  the  feeding 
of  debilitated  persons ;  by  frequent  feeding  with  appetizing  food 
the  nutritional  condition  is  likely  to  improve  much  more  rapidly 
than  by  occasional  stuffing  with  uncongenial  mixtures,  however 
rich  these  may  be  in  calories  and  nitrogen. 

The  secretion  is  therefore  well  named  the  appetite  juice,  and 
it  lasts  sometimes  for  nearly  two  hours  after  sham  feeding  has 
been  discontinued.  Yet  this  is  only  about  one-half  as  long  as  the 
time  during  which  gastric  juice  is  secreted  when  the  food  is  ae- 


Fig.  6. — Diagram  of  stomach  showing  miniature  stomach  (S)  separated 
from  the  main  stomach  (Y)  by  a  double  layer  of  mucous  membrane.  A. A.  is 
the   opening   of   the   pouch    on   the   abdominal   wall.      (Pavlov.) 


tually  permitted  to  enter  the  stomach.  In  order  to  investigate 
the  cause  of  tJie  continued  secretion,  it  was  necessary  to  devise 
some  means  by  which  the  gastric  juice  could  be  collected,  un- 
mixed with  food,  while  normal  digestion  was  in  progress.  As 
there  is  no  duct,  the  only  means  by  which  this  could  be  done  was 
by  isolating  a  portion  of  the  stomach  as  a  pouch  with  an  opening 
exteriorly  through  which  the  secretions  collecting  in  it  could  be 
removed.  An  operation  for  making  such  a  pouch,  or  "miniature 
stomach,"  as  it  is  called,  without  injuring  any  of  the  nerves  of 


DIGESTION    IN    THE   STOMACH.  63 

the  stomach  has  been  'devised  by  Pavlov  (see  Fig.  6).  By  sim- 
ultaneously 'collecting  the  secretions  from  the  main  stomach  and 
the  miniature  stomach  after  sham  feeding,  it  was  found  that  they 
ran  strictly  parallel  with  each  other,  in  amount  as  well  as  in 
strength  of  secretion.  The  secretion  in  the  miniature  stomach 
therefore  accurately  mirrors  the  secretion  occurring  in  the  main 
stomach,  and  so  permits  us  to  study  this  during  the  actual  diges- 
tion of  food. 

By  introducing  food  directly  into  the  main  stomach  through 
a  fistula,  it  was  found,  by  observations  on  the  secretions  from  the 
miniature  stomach,  that  very  little  secretion  occurred  until  after 
some  time,  provided  of  course  that  precautions  had  been  taken, 
as  by  experimenting  on  a  sleeping  animal,  not  to  excite  the  appe- 
tite juice.  There  was  found  to  be  great  discrimination  in  the 
nature  of  the  adequate  stimulus  for  this  local  secretion ;  mechani- 
cal stimulation  of  the  gastric  mucosa,  contact  with  alkaline  fluids, 
such  as  saliva,  or  with  white  of  egg,  failed  to  produce  any  secre- 
tion ;  water  had  a  slight  effect,  milk  still  more,  whereas  a  marked 
secretion  occurred  when  a  decoction  of  meat  or  meat  extract,  or 
a  solution  containing  the  half-digested  products  of  peptic  diges- 
tion (such  as  Witte's  peptone)  was  placed  in  the  main  stomach. 
It  was  further  observed,  when  meat  was  directly  placed  in  the 
stomach,  that  the  juice  which  collected  in  the  pouch  increased, 
both  in  quantity  and  in  strength,  after  the  first  hour,  and  that 
it  continued  to  flow  even  after  four  hours,  thus  indicating  that 
the  primary  stimulus  had  come  from  the  extractives  in  the  meat, 
further  stimulation  being  due  to  the  proteose  and  peptones 
liberated  as  the  protein  of  the  meat  became  digested. 

This  local  stimulation  is  independent  of  the  medullary  nerve 
center  that  controls  secretion  of  the  appetite  juice,  for  it  still  oc- 
curred after  both  vagi  had  been  divided  or  even  after  destruc- 
tion of  the  sympathetic  nerve  plexuses  in  the  abdomen.  It  might, 
however,  still  be  a  nervous  reflex  involving  the  local  nerve  struc- 
tures (plexus  of  Auerbach)  in  the  walls  of  the  stomach,  although 
this  is  not  so  probable  as  that  it  is  dependent  upon  some  chemical 
excitation  of  the  gland  cells  by  substances  appearing  in  the  blood 


64  HUMAN   PHYSIOLOGY. 

as  a  result  of  absorption  from  the  stomach.  This  "hormone" 
(see  p.  124)  is  not  merely  absorbed  food,  for  no  gastric  secretion 
occurred  when  solutions  of  meat  extract,  or  of  peptone  were  in- 
jected intravenously.  It  must  therefore  be  some  substance  which 
is  absorbed  into  the  blood  from  the  mucous  membrane  of  the 
stomach,  and  which  is  produced  in  this  as  a  result  of  the  action 
of  the  gastric  contents  on  its  cells.  In  confirmation  of  this  view 
it  has  been  shown  that  boiled  extracts  of  the  mucous  membrane 
of  the  pyloric  region  of  the  stomach  (made  with  water  or  weak 
acid  or  solutions  of  peptone  or  dextrin.)  cause  some  gastric  juice 
to  be  secreted  when  they  are  injected  in  small  quantities  every 
ten  minutes  into  a  vein,  similar  injections  of  the  extracting  fluids 
themselves  being  without  effect. 

"We  are  now  provided  with  the  necessary  facts  from  which  to 
draw  a  completed  account  of  the  mechanism  of  gastric  secretion. 
The  satisfaction  of  taking  food  causes  appetite  juice  to  flow  and 
this  soon  digests  some  of  the  protein.  The  products  of  this  diges- 
tion, along  with  the  extractive  substances  of  the  food,  after  some 
time  (which  is  probably  quite  short  in  the  case  of  man),  gain  the 
pylorus,  where  they  act  on  the  mucosa  to  produce  some  hormone, 
which  becomes  absorbed  into  the  blood  and  stimulates  further 
secretion  of  the  juice.  As  digestion  proceeds  juice  therefore  con- 
tinues to  be  secreted.  The  appetite  juice  sets  the  process  agoing ; 
it  initiates  gastric  digestion. 

The  Active  Constituents  of  Gastric  Juice. — "When  there  is  no 
food  in  the  stomach,  a  certain  amount  of  mucous  secretion 
is  present  in  it,  and  most  of  the  gland  cells  are  filled  with  zymo- 
gen granules  (see  p.  40).  An  extract  (made  with  glycerine) 
of  the  mucosa  in  this  resting  condition  exhibits  no  digestive 
powers;  but  if  the  mucosa  be  first  of  all  macerated  with  weak 
hydrochloric  acid,  the  extract  becomes  highly  active,  because  it 
contains  large  amounts  of  the  proteolytic  ferment  pepsin.  Other 
cells  in  the  stomach  produce  the  necessary  hydrocliloric  acid. 
It  may  be  concluded,  therefore,  that  during  the  process  of  secre- 
tion the  zymogen  granules  are  activated  by  hydrochloric 
acid  and  converted  to  pepsin.  In  conformity  with  this,  it 
has  been  found  that  the  secretion  of  a  pouch  of  stomach  pre- 


DIGESTION   IN   THE    STOMACH.  65 

pared  from  the  pyloric  region  possesses  no  digestive  activity, 
since  in  this  region  no  hydrochloric  acid  is  secreted.  The  activa- 
tion of  the  zymogen  can  also  be  accomplished  by  tissue  extracts 
and  by  the  products  of  miero-organismal  growth.  Because  of 
such  growth  in  the  stomach  contents,  it  is  often  found,  in  dis- 
eased conditions  in  which  there  is  no  acid  secretion,  that  active 
pepsin,  nevertheless,  is  present.  Accompanying  the  pepsin,  if 
indeed  not  identical  witli  it,  the  gastric  juice  contains  the  milk- 
curdliug  ferment,  rennin.  It  also  contains  a  fat-splitting  fer- 
ment, lipase,  whose  activities  are,  however,  limited  to  emulsified 
fats. 

The  most  remarkable  constituent  of  the  gastric  secretion  is* 
hydrocliloric  acid,  which  in  some  animals,  such  as  the  dog,  may 
attain  a  percentage  of  0.6,  being  usually  about  0.4  in  the  ease  of 
man.  It  is  derived  from  the  parietal  cells  of  the  glands  in  the 
cardiac  region  of  the  stomach,  none  being  present  in  the  secre- 
tion of  the  pyloric  region,  where  there  are  no  parietal  cells. 

The  source  of  the  acid  is  of  course  the  blood,  for  although  this 
is  practically  neutral,  yet  it  contains,  on  the  one  hand,  substances 
such  as  sodium  bicarbonate  which  readily  yield  hydrogen  ions, 
and  on  the  other,  chlorides  which,  by  dissociation,  make  chlorine 
ions  readily  available.  Although  it  is  thus  possible,  in  the  light 
of  modern  physico-chemical  teaching,  to  formulate  an  equation 
for  the  reaction,  yet  we  are  at  a  loss  to  explain  why  just  at  this 
particular  place  (i.  e.,  in  the  gland  cells  of  the  stomach)  in  the 
animal  body,  and  nowhere  else,  the  CI-  and  H-ions  should  be 
picked  out  of  the  blood  and  secreted  as  HCl. 

Little  as  we  know  about  the  cause  and  mechanism  of  the  secre- 
tion of  hydrochloric  acid,  we  do  know  something  regarding  its 
value  and  use  in  the  process  of  digestion,  and  in  general  we  may 
state  that  this  is  partly  regulatory  and  partly  digestive.  ,  It  is 
regulatory  in  that  it  serves  as  the  exciting  cause  of  subsequent 
events  in  the  digestive  process,  and  digestive  not  only  in  that  it 
actually  assists  in  the  break-down  of  protein,  but  also  because  it 
may  cause  a  certain  amount  of  acid  hydrolysis  of  sugar  after 
enough  has  been  secreted  so  that  some  is  free.  Its  action 
on  protein  is,  however,  the  most  important,  for  it  initiates  pro- 


66  HUMAN   PHYSIOLOGY. 

teolytic  break-down  by  producing  so-called  acid — protein  on 
which  the  pepsin — itself  also  dependent,  as  we  have  seen,  on  a 
preliminary  activation  by  acid — then  unfolds  its  action.  As  the 
protein  becomes  progressively  broken  down,  the  proteose  and 
peptone  which  are  produced  absorb  still  more  of  the  acid,  so 
that  it  is  some  considerable  time  after  gastric  digestion  has 
started  before  any  acid  is  lallowed  to  exist  in  the  free  state. 

It  is  only  after  there  is  some  free  acid  that  it  can  hydrolyse 
sugars  or  perform  another  important  function,  namely,  act  as  an 
antiseptic.  In  this  regard,  however,  it  must  be  remembered  that 
it  is  only  towards  certain  organisms  that  such  antiseptic  action 
is  displayed,  for  there  may  be  bacteria  in  the  gastric  contents 
even  in  cases  of  excessive  secretion  of  hydrochloric  acid.  The 
undoubted  tendency  for  intestinal  putrefaction  to  increase  when 
there  is  a  deficient  secretion  of  hydrochloric  acid  is  probably  de- 
pendent more  upon  the  delay  in  digestion  which  this  occasions, 
than  upon  any  specific  antiseptic  power  of  hydrochloric  acid. 
During  the  time  that  elapses  before  a  sufficiency  of  hydrochloric 
acid  has  accumulated  to  perform  this  function,  bacterial  fermen- 
tation occurs  in  the  stomach  contents.  Carbohydrates  are  broken 
down  by  this  process,  at  .first  into  simple  sugars  and  then  into 
lactic  acid,  which  may  come  to  be  present  in  considerable  amount 
before  the  fermentation  process  is  terminated.  For  these  reasons 
we  find  that  there  is  relatively  much  more  lactic  acid  detectable 
in  the  gastric  contents  removed  by  the  stomach  tube  at  an  early 
stage  in  gastric  digestion  than  later. 

The  so-called  acid  albumin  which  results  from  the  action  of 
the  acid,  becomes  attacked  by  the  pepsin,  which  still  further 
breaks  it  down  into  so-called  proteose  and  peptones,  which  do 
not  coagulate  by  heat  and  which  become  progressively  more  dif- 
fusible through  animal  membranes.  Although  pepsin  is  capable 
of  carrying  the  digestive  process  far  beyond  the  stage  of  pep- 
tones, this  does  not  occur  in  the  comparatively  short  time  (about 
six  hours)  during  which  the  food  remains  in  the  stomach.  Slight 
as  is  this  action  of  pepsin  in  the  stomach,  it  nevertheless  appears 
to  be  of  considerable  importance  for  the  subsequent  digestion  of 
protein  by  the  other  proteolytic  ferments,  trypsin  and  erepsin 
(see  p.  75),  which  operate  in  the  small  intestine.     Thus,  a  given 


DIGESTION    IN   THE    STOMACH. 


67 


amount  of  blood  serum  becomes  digested  much  farther  in  a 
given  time  by  a  given  amount  of  trypsin  if  it  receives  a  prelim- 
inary digestion  b}^  means  of  pepsin,  than  when  it  is  acted  on  by 
trypsin  alone,  and  erepsin  will  cause  no  digestion  of  most  pro- 
teins unless  these  are  first  of  all  acted  on  by  either  pepsin  or 
trypsin.  But  peptic  digestion  is  not  essential  for  life,  for  sev- 
eral cases  are  now  on  record  in  which  individuals  have  thrived 
after  the  stomach  has  been  removed. 

The  milk  curdling  action  of  gastric  juice  is  due  partly  to  the 
hydrochloric  acid  and  partly  to  pepsin.  Curiously  enough  the 
curdled  milk  undergoes  little  further  change  until  it  reaches 
the  small  intestine. 

The  lipase  in  gastric  juice  can  act  only  on  emulsified  fat  and 
in  a  neutral  or  alkaline  reaction.  Fat  digestion  cannot  therefore 
be  an  important  gastric  process. 

It  has  been  supposed  that  there  is  a  certain  specific  adaptation 
between  the  chemical  nature  of  the  food  and  the  amount  and 
strength  of  the  gastric  secretion.  For  example,  it  has  been 
found,  by  observations  on  the  gastric  juice  flowing  from  a  minia- 
ture stomach  (see  Fig.  6),  that  feeding  with  bread  causes  a 
maximal  secretion  during  the  first  hour,  whereas  with  an  equiva- 
lent amount  of  flesh  the  maximum  occurs  during  the  first  and 
second  hours,  and  with  milk  it  is  delayed  till  the  third  or  fourth. 
In  proteolytic  power  the  bread  juice  is  much  the  strongest  of  the 
three,  but  it  contains  a  lower  percentage  of  acid  than  the  others. 

The  Movements  of  the  Stomach. — Solid  food  after  being 
swallowed  accumulates  in  the  body  of  the  stomach,  where  on  ac- 
count of  an  absence  of  movements  it  is  not  uniformly  acted  on 
by  the  gastric  juice,  its  outer  layers  only  becoming  digested.  In 
the  case  of  man,  however,  some  of  the  food,  because  of  its 
semi-fluid  nature,  passes  beyond  the  so-called  transverse  band 
and  into  the  pyloric  region,  in  which  waves  of  contraction  make 
their  appearance.  Starting  very  faintly  at  this  point,  these 
waves  travel  towards  the  pylorus  and  become  gradually  more 
marked  until  they  may  become  so  deep  as  practically  to  cut  off  a 
portion  of  the  pyloric  region  from  the  rest  of  the  stomach.  This 
last  portion  of  the  pylorus,  sometimes  called  the  pyloric  canal, 


68  HUMAN   PHYSIOLOGY. 

gradually  contracts  on  the  food  which  has  been  forced  into  it, 
thus  tending  to  eject  it  through  the  pyloric  sphincter,  or,  if  this 
is  closed,  to  cause  it  to  pass  back  again  as  an  axial  stream  into 
the  proximal  part  of  the  pylorus,  which  has  been  called  the 
pyloric  vestibule.  These  waves  occur  every  fifteen  to  twenty 
seconds,  three  or  four  being  present  in  the  pyloric  vestibule  at 
one  time.  They  become  more  marked  as  digestion  proceeds,  and 
are  accompanied  by  a  gradual  diminution  in  size  of  the  body 
of  the  stomach  (see  Fig.  5).  Their  function,  besides  carrying 
the  food  towards  the  outlet  of  the  stomach,  is  to  keep  it  properly 
mixed  with  the  gastric  juice. 

The  Opening'  of  the  Pyloric  Sphincter. — The  mere  pressure 
with  which  the  contents  of  the  vestibule  are  thus  driven,  with 
each  peristaltic  wave,  against  the  pyloric  sphincter  does  not 
alone  succeed  in  opening  it ;  for  half  an  hour  after  feed- 
ing with  protein,  for  example,  no  food  may  pass  the  sphincter, 
although  during  this  time  there  may  have  been  well  over  a  hun- 
dred peristaltic  waves.  Nor  is  it  the  consistency  of  the  food 
which  controls  the  opening.  It  must  therefore  be  some  chemical 
property  which  the  food  acquires  during  its  stay-  in  the  stomach. 
This  has  definitely  been  shown  by  Cannon  to  be  the  presence  of 
free  acid.  By  measuring  the  length  of  the  skiagram  shadow  in 
the  intestines  after  feeding  cats  with  bismuth-impregnated  foods 
rendered  acid  or  alkaline,  it  could  be  clearly  shown  that  acid 
hastened  the  initial  discharge,  whereas  alkalies  retarded  it,  and 
observations  through  a  fistula  in  the  vestibule  showed  that  any 
delay  in  the  appearance  of  acid  in  the  contents  was  associated 
with  a  delay  in  the  opening  of  the  sphincter. 

But  the  sphincter  does  not  remain  open;  it  quickly  closes 
after  a  little  chyme,  as  the  half  digested  food  is  called,  has  passed 
through  it.  This  closure  is  due  to  stimulation  of  afferent  nerve 
endings  in  the  duodenum  by  the  free  acid  in  the  chyme.  The 
sphincter  remains  closed  so  long  as  there  is  any  free  acid  in  the 
duodenum.  Whenever  this  acidity  has  -become  neutralized  by 
the  alkali  present  in  the  bile  and  pancreatic  juice,  the  acid  on 
the  stomach  side  again  becomes  operative  and  the  sphincter 
opens. 


DIGESTION   IN   THE   STOMACH.  69 

The  pyloric  sphincter  is  thus  under  the  control  of  a  nerve 
reflex,  called  the  duodenal  reflex,  fvhich  transmits  influences 
that  tend  to  relax  the  sphincter  when  the  afferent  nerve  fibers 
from  the  stomach  side  are  excited  by  acid,  but  which  cause  it  still 
more  powerfully  to  contract  when  the  acid  acts  on  afferent  fibers 
having  their  terminations  in  the  duodenum.  When  both  afferent 
paths  are  simultaneously  stimulated,  the  duodenal  predominates 
over  the  gastric,  so  that  the  sphincter  remains  closed  until  the 
acidity  of  the  chyme  in  the  duodenum  has  all  been  neutralized, 
and  this  seems  to  be  true  however  faint  the  acidity  may  be  on 
the  duodenal  side  and  however  strong  on  the  stomach  side.  The 
reflex  arc  is  situated  in  the  w^alls  of  the  stomach  and  duodenum, 
for  it  operates  after  complete  isolation  of  these  from  the  central 
nervous  system.  It  is  a  function  of  the  nerve  plexus  found  pres- 
ent in  the  walls — the  myenteric  plexus. 

Rate  of  Discharge  of  Food  from  the  Stomach. — The  acidity 
of  the  gastric  contents,  as  we  have  just  seen,  must  attain  a  cer- 
tain degree  before  it  becomes  an  adequate  stimulus  for  the  open- 
ing of  the  pyloric  sphincter,  and  consequently  the  rate  at  which 
the  different  foodstuffs  leave  the  stomach  is  to  a  large  extent 
proportional  to  their  power  of  combination  with  the  acid.  Pro- 
teins combine  with  large  amounts  of  acid,  so  that  their  initial 
discharge  is  delayed  and  their  subsequent  passage  slow.  Car- 
bohydrates absorb  but  little  acid,  so  that  they  begin  to  leave  early 
and  the  stomach  is  soon  emptied  of  them.  The  passage  of  fats  is 
peculiar;  when  taken  alone,  which,  however,  is  scarcely  ever  the 
case,  they  seem  to  bring  about  a  partial  relaxation  of  the  pyloric 
sphincter,  so  that  bile  and  pancreatic  juice  regurgitate  into  the 
stomach  and  .some  fat  may  pass  out;  but  the  subsequent  dis- 
charge into  the  intestines  is  very  slow,  so  slow  indeed  that  each 
discharged  portion  seems  to  become  completely  absorbed'  before 
any  furth(!r  discharge  occurs.  When  fats  are  mixed  with  other 
foofls,  they  materially  delay  the  discharge.  These  effects  are 
no  dou])t  due  in  i)art  to  the  inhibitory  influence  which  fats  have 
on  gastric  secretion ;  and  in  part  to  the  liberation  of  fatty  acid  in 
the  duodenum  by  the  action  of  pancreatic  lipase.    This  fatty  acid 


70  HUMAN   PHYSIOLOGY. 

seems  to  be  liberated  so  quickly  that  it  is  not  immediately  neu- 
tralized by  alkali. 

Water  alone  begins  to  leave  the  stomach  almost  immediately 
after  it  is  taken,  because  in  this  case  the  sphincter  opens  before 
an  acid  reaction  has  been  acquired,  and  remains  open  on  account 
of  there  being  no  acid  in  the  duodenum  to  effect  its  closure. 
Water  does  not  remain  for  a  sufficient  time  in  the  stomach  to 
excite  any  gastric  secretion,  and  consequently  it  readily  car- 
ries infection  into  the  intestine.  The  discharge  of  raw  egg  al- 
bumin is  peculiar.  Like  water  it  begins  to  pass  the  pylorus  im- 
mediately after  ingestion,  its  reaction  for  some  time  being  alka- 
line ;  it  becomes  acid  later,  so  that  the  discharge  becomes  inter- 
mittent because  of  the  -duodenal  reflex.  The  consistency  of  food 
itself  does  not  affect  the  rate  of  discharge  unless  hard  particles 
are  present  in  it,  when  a  marked  retardation  occurs. 

It  is  well  known  that  the  gastric  contents  are  but  slowly  dis- 
charged into  the  duodenum  when  there  is  excessive  gas  accu- 
mulation in  the  stomach.  This  is  due  to  the  atony  of  the  stomach 
which  accompanies  pathological  gas  accumulation. 


CHAPTER  VII. 

DIGESTION  (Cont'd). 

Intestinal  Digestion:    The  Movements  of  the  Intestines: 
Absorption, 

The  Secretion  of  Bile  and  Pancreatic  Juice. — Besides  caus- 
ing reflex  closure  of  the  pyloric  sphincter,  the  contact  of  the 
chyme,  which  is  the  name  given  to  the  semi-digested  food  as  it 
leaves  the  stomach,  with  the  duodenal  mucosa  inaugurates  the 
processes  of  intestinal  digestion  by  exciting  the  secretion  of  bile 
and  pancreatic  juice.  Neither  of  these  juices  is  secreted  into  the 
intestine  during  fasting;  but  both  begin  to  flow  very  soon  after 
taking  food,  and  they  gradually  increase  in  amount  for  about 
three  hours,  and  then  rapidly  decline.  The  bile  at  first  comes 
mainly  from  the  gall  bladder,  in  which  it  has  accumulated  dur- 
ing fasting.  When  the  gall  bladder  supply  has  been  exhausted, 
the  bile  comes  directly  from  the  liver  without  entering  the  gall 
bladder.  This  direct  secretion  becomes  more  and  more  marked 
as  digestion  proceeds. 

Bile  is  partly  an  excretory  product  of  the  liver,  and  is  thus 
being  constantly  secreted  into  the  bile  ducts.  On  account  of  its 
value  as  a  digestive  fluid  it  is  not,  however,  allowed  to  run  to 
waste,  but  is  stored  up  in  the  gall  bladder  until  food  arrives  in 
the  duodenum,  when  the  bile  is  immediately  discharged  as  above 
described. 

The  sudden  discharge  of  bile  from  the  gall  bladder  is  depen- 
dent upon  a  nerve  reflex  excited  by  the  contact  of  the  acid  chyme 
with  the  duodenum.  The  increased  secretion  of  bile  which  occurs 
later  in  digestion,  like  the  secretion  of  pancreatic  juice,  is,  how- 
ever, independent  of  nerves,  for  it  has  been  found  that  it  occurs 
when  acid  is  plac(;d  in  the  duodenum  after  all  the  nerves,  but 
not  the  blood  vessels  of  the  duodenum,  have  been  cut.  The 
only  way  by  which  such  a  result  can  be  explained  is 
by     assuming     that     the     acid     causes     some     chemical     sub- 

71- 


72  HUMAN   PHYSIOLOGY. 

stance  to  be  added  to  the  blood,  which  then  carries  it  to  the  pan- 
creas and  liver,  upon  the  cells  of  which  it  exercises  a  stimulating 
influence.  This  explanation  was  shown  to  be  correct  by 
studying  the  effect  which  is  produced  on  the  secretion  of  pan- 
creatic juice  and  bile  by  intravenous  injections  of  decoctions  of 
intestinal  mucosa  made  with  weak  acid  and  subsequently  neu- 
tralized. An  immediate  secretion  resulted.  The  acid  extract 
contains  some  hormone  whose  production,  in  the  normal 
process  of  digestion,  is  evidently  occasioned  by  the  contact  of  the 
acid  chyme  with  the  duodenal  mucosa.  This  hormone  is  called 
secretin  but  we  know  very  little  of  its  exact  chemical  nature.  It 
is  not  a  ferment,  for  it  withstands  heat ;  it  is  not  a  protein,  for  it 
can  be  extracted  by  boiling  the  mucous  membrane  with  weak 
acids  after  treatment  with  alcohol.  It  is  readily  oxidized  in  the 
presence  of  alkalies,  and  is  of  the  same  nature  in  all  animals. 
It  is  useless  to  give  secretin  as  a  drug  with  the  hope  that  it  will 
stimulate  pancreatic  secretion,  for  it  is  not  absorbed  from  the 
lumen  of  the  intestine. 

Although  most  abundant  in  the  mucosa  of  the  duodenum  and 
jejunum,  secretin  is  also  present  in  the  mucosa  of  the  lower  end 
of  the  small,  and  to  a  lesser  degree,  in  that  of  the  large  intestine. 
Soap  solutions  act  like  acid  in  producing  secretin.  A  fatty  meal, 
therefore,  excites  the  flow  of  much  pancreatic  juice  and  bile,  be- 
cause the  fatty  acid  which  is  split  off  unites  with  alkali  and 
forms  soap. 

It  may  be  that  the  first  portion  of  pancreatic  juice  to  be  se- 
creted after  ,a  meal,  is  the  result,  not  of  secretin  formation,  but 
of  reflex  nervous  stimulation  of  the  pancreas.  In  comparison 
with  the  hormone  control  the  nervous  control  is,  however,  quite 
unimportant  in  pancreatic  secretion,  for  there  is  no  necessity  in 
the  intestine^  as  in  the  mouth,  or  to  a  less  degree  in  the  stomach, 
for  a  quick  response  to  the  stimulus  which  is  set  up  by  the  pres- 
ence of  food.  The  histological  changes  produced  in  the  gland 
cells  of  the  pancreas  by  secretory  activity  are  much  the  same  as 
in  the  parotid  glands. 

Functions  of  the  Bile  and  Pancreatic  Juice. — These  two 
juices  are  very  closely  associated  in  their  activities.     This  fact 


INTESTINAL  DIGESTION.  73 

is  perhaps  most  strikingly  demonstrated  in  the  digestion  and  ab- 
sorption of  fat;  for,  in  the  absence  of  either  secretion,  large 
amounts  of  unabsorbed  fat  appear  in  the  faeces.  Both  juices 
contain  relatively  large  amounts  of  alkali,  ivhich  neutralizes  the 
acidity  of  the  chyme.  In  the  pancreatic  juice  alone,  for  example, 
there  is  a  sufficient  concentration  of  sodium  carbonate  to  neu- 
tralize the  acid  in  an  equal  volume  of  gastric  juice.  Whenever 
the  chyme  becomes  alkaline  the  pepsin  present  in  it  ceases  to  act 
and  conditions  thus  become  suitable  for  the  activities  of  the  pan- 
creatic enzymes.  Besides  its  neutralizing  action,  the  bile  causes 
the  chyme  to  assume  a  somewhat  greater  consistency,  by  pre- . 
cipitating  incompletely  peptonized  protein,  as  well  as  pepsin. 
The  precipitate  becomes  redissolved  when  excess  of  bile  has  be- 
come mixed  with  the  chyme  and  the  significance  of  the  precipita- 
tion may  be  that  it  causes  a  temporary  delay  in  the  movement  of 
the  chyme  along  the  duodenum,  thus  allowing  it  to  become  prop- 
erly mixed  with  pancreatic  juice  before  it  moves  further  along 
the  intestine. 

Composition,  Properties  and  Functions  of  the  Bile. — 

Water 85.9 

Total  Solids 14.1 

of  which : 


Organic  < 


IJile  Salts 0.11 

[jccithin  and  Cholesterol 1.10 

Mucinoid  Substar.ce     ,  „  „ 
Pigment 


Iiioi'gaiiic   Salts   0.78 

The  bile  is  a  greenish-yellow  fluid  of  sticky  consistency  and 
bitter  taste.  Its  most  interesting  constituents  are  the  bile  salts, 
which  are  complex  organic  substances,  having  an  important  func- 
tion to  perform  in  assisting  the  lipase  and  amylopsin  of  pan- 
creatic juice  in  their  digestive  activities.  Otherwise  the  bile  con- 
tains no  digestive  enzymes.  The  cholesterol  is  not  a  readily 
soluble  substance,  so  that  it  is  ai)t  to  become  precipitated  in  the 


74  HUMAN   PHYSIOLOGY, 

bile  duct  and  cause  gall  stones.  The  distention  of  tlie  ducts 
by  the  gall  stones  may  cause  great  pain  (biliary  colic).  The 
formation  of  gall  stones  is  encouraged  by  inflammatory  processes 
of  the  mucous  membrane  of  the  ducts.  When  bile  fails  to  reach 
the  intestine,  because  of  blocking  the  ducts,  either  by  gall  stones 
or  by  inflammatory  swelling  of  the  mucous  membrane,  the  di- 
gestion, especially  of  fats,  is  much  interfered  with/  and  the  f sees 
become  foul  smelling  and  pale  in  color. 

The  Composition  and  Properties  of  Pancreatic  Juice. — The 
pancreatic  juice  contains  three  important  enzymes:  lipase  (act- 
ing on  fats),  amylopsin  (acting  on  starch),  and  trypsinogen 
(acting  on  protein).  Lipase  and  amylopsin  are  secreted  in  an 
active  condition,  but  trypsinogen  is  without  any  action  until  it 
has  become  changed  into  trypsin.  This  does  not  occur  until  the 
pancreatic  juice  has  reached  the  intestine,  when  the  activation 
is  brought  about  by  a  ferment  present  in  the  intestinal  juice 
(secretion  of  Lieberkiihn 's  follicles)  called  enteroMnase.  The 
intestinal  juice  contains  this  activator  only  when  there  is  some 
trypsinogen  present  in  the  intestine.  There  is  no  enterokinase, 
fbr  example,  in  the  juice  that  is  secreted  as  a  result  of  mechan- 
ical stimulation  of  the  intestinal  mucosa,  but  it  immediately  ap- 
pears when  some  pancreatic  secretion  is  brought  in  contact  with 
the  mucosa. 

Enterokinase  is  not  the  only  substance  which  can  activate 
trypsinogen;  the  addition  to  the  pancreatic  juice  of  calcium 
salts,  or  the  contact  of  the  juice  with  leucocytes,  as  in  granula- 
tion tissue,  or  even  mere  standing  of  the  juice,  has  a  similar  ac- 
tivating effect.  If  the  pancreatic  juice,  in  escaping  from  the  duet, 
should  run  over  granulation  tissue,  as  occurs  when  a  fistula 
(i.  e.,  an  opening  made  by  surgical  operation)  of  the  duct  is 
made,  it  becomes  activated  and  unless  precautions  are  taken  it 
will  excoriate  the  wound.  Should  it  escape  into  the  peritoneum, 
as  when  a  cyst  bursts,  it  also  becomes  activated. 

It  will  be  remembered  that  the  amount  of  gastric  juice  secreted 
varies  with  different  foods,  being  relatively  more  abundant  on  a 
diet  of  bread  than  on  one  of  milk,  or  even  meat  (p.  63).     Simi- 


INTESTINAL  DIGESTION.  75 

lar  quantitative  differences  exist  in  the  secretion  of  pancreatic 
juice  and  this  is  probably  to  be  explained  by  the  varying  quanti- 
ties of  acid  chyme  coming  in  contact  with  the  duodenal  mucosa. 

Chemical  Changes  Produced  by  Intestinal  Digestion. — In  the 
lower  portion  of  the  duodenum  and  in  the  jejunum,  the  digestive 
enzymes  of  the  pancreatic  juice  act  on  the  food  in  full  intensity. 
The  trj^psin  rapidly  hydrol3^zes  the  proteins  to  peptone,  which  if 
it  is  not  immediately  absorbed  may  become  further  broken  down 
to  amino  acids  and  aromatic  compounds.  The  lipase  hydrolyses 
fat  to  glycerine  and  fatty  acid,  which  are  absorbed,  the  former 
as  such,  the  latter,  after  combining  with  alkali  to  form  soap,  or, 
if  no  alkali  be  available,  with  bile  salts  to  form  compounds  which 
like  soap  are  soluble  in  water.  Amylopsin  converts  into  mal- 
tose any  starch  or  dextrines  which  the  ptyalin  of  saliva  has  failed 
to  act  on.  The  maltose  thus  formed,  and  the  other  disaccharides, 
cane  sugar  and  lactose,  although  soluble  in  water,  do  not  become 
'absorbed  into  the  blood  as  such  but  become  further  hydrolyzed 
by  the  action  of  so-called  inverting  enzymes,  of  which  there  is  one 
for  each  disaccharide  (see  p.  25).  These  inverting  enzymes  are 
more  plentiful  in  extracts  of  the  mucosa  than  in  the  intestinal 
juice  itself,  from  which  we  conclude  that  it  is  only  after  they 
have  been  absorbed  into  the  cells  of  the  intestines  that  the  disac- 
charides are  inverted.  The  process,  in  other  words,  is  an  in- 
tracellular one. 

One  other  enzyme  exists  in  the  intestinal  juice,  nsLmely, crepsin. 
It  acts  on  partially  hydrolyzed  proteins  and  on  caseinogen,  so 
as  to  hydrolyzc  them  completely  into  the  amino  acids. 
Erepsin  is  a  widely  distributed  enzyme  in  the  animal  body,  be- 
ing present  in  practically  every  tissue,  although  it  is  absent  from 
blood  plasma.  It  is  present  in  much  greater  concentration  in  ex- 
tracts of  the  intestinal  mucosa  than  in  the  succus  entericus,  so 
that,  like  the  inverting  enzymes,  it  possibly  displays  its  action 
while  the  protein  is  being  absorbed  as  proteoses  and  peptones. 
It  serves  as  the  last  barrier  against  the  entry  into  the  blood  of 
protein  in  any  other  form  than  as  a  mixture  of  amino  acids.  Less 
completely  digested  protein  is  poisonous  when  added  to  the  blood 
(p.  152). 


76  HUMAN  PHYSIOLOGY. 

Most  of  the  food  is  now  in  a  suitable  condition  for  absorption. 
Before  we  proceed  to  study  the  nature  of  this  process,  however, 
there  are  one  or  two  further  digestive  changes  that  we  must  con- 
sider. 

The  Digestive  Function  of  Intestinal  Bacteria. — On  account 
of  the  antiseptic  action  of  free  hydrochloric  acid,  there  is,  ordi- 
narily, no  bacterial  growth  in  the  stomach,  but  the  neutraliza- 
tion of  acid  by  the  pancreatic  juice  and  bile  in  the  intestine  pro- 
vides a  perfect  medium  for  such  growth.  The  extent  and  nature 
of  the  bacterial  growth  varies  very  greatly  according  to  the  na- 
ture of  the  diet. 

There  can  be  no  doubt  that  the  micro-organisms  are  a  valuable 
aid  to  digestion  in  the  case  of  most  animals,  especially  of  those 
whose  diet  includes  cellulose.  Indeed,  in  such  animals  as  the 
herbivora  special  provision  is  made  to  encourage  bacterial  growth 
by  the  great  length  of  the  large  intestine,  for  without  bacteria, 
digestion  of  cellulose  is  impossible.  Thus  if  newly-hatched  chicks 
be  fed  with  sterilized  grain  they  succumb  in  about  two  weeks, 
but  if  a  small  amount  of  the  excrement  of  the  fowl  be  mixed 
with  the  grain,  they  thrive  as  ordinarily.  On  the  other  hand,  if 
the  food  contains  no  cellulose,  animals  may  develop  and  grow 
with  sterile  intestinal  contents;  thus  guinea  pigs  have  been  re- 
moved from  the  uterus  under  aseptic  conditions  and  kept  in  a 
sterile 'place  on  sterilized  milk  and  have  thrived  and  grown  as 
normal  guinea  pigs.  The  organisms  in  the  intestine  of  man  are 
probably  much  more  useful  than  harmful.  No  doubt  they  are 
parasites,  but  they  are  useful  parasites ;  they  work  for  their  liv- 
ing, not  only  by  assisting  when  necessary  in  the  digestion  of 
food  but  also  by  destroying  certain  substances  which,  if  absorbed, 
would  have  a  toxic  action  on  the  host.  Thus  cholin,  a  substance 
produced  by  the  digestion  of  lecithin,  is  distinctly  poisonous,  but 
it  really  never  gets  into  the  blood  because  the  bacteria  destroy  it. 

In  the  case  of  man  bacterial  digestion  occurs  in  both  the  small 
and  the  large  intestines,  and  there  are  varieties  of  bacteria  capa- 
ble of  acting  on  all  the  foodstuffs.  They  may  break  up  the  sugars 
into  lactic  acid  or  even  further  so  as  to  form  CO,  and  H.  It  has 
been  claimed  that  this  formation  of  lactic  acid  in  the  intestine  is 


INTESTINAL  DIGESTION.  77 

of  benefit  to  the  health  of  man  because  when  it  occurs  other  bac- 
teria which  are  more  harmful  than  useful  become  destroyed.  To 
encourage  this  growth  of  lactic  acid  bacteria,  it  has  been  recom- 
mended that  large  quantities  of  sour  milk  sliould  be  taken.  It  is 
undoubtedly  true  that  such  treatment  is  of  benefit  in  many  per- 
sons who  suffer  from  excessive  intestinal  putrefaction,  but  that 
such  treatment  should  prolong  the  life  of  otherwise  healthy  indi- 
viduals is  visionary.  As  in  herbivora,  there  are  also  bacteria  in 
man  which  break  up  cellulose,  producing  methane  and  CO,.  After 
diets  containing  much  vegetable  matter,  therefore,  a  large 
amount  of  gas  is  likely  to  accumulate  in  the  intestines.  From 
fats,  the  intestinal  bacteria  produce  lower  fatty  acids,  which 
tend  to  cause  the  contents  in  the  lower  portion  of  the  small  in- 
testines to  become  acid  in  reaction. 

Although  capable  of  hydrolyzing  native  protein  from  the  very 
start,  bacteria  act  most  readily  on  protein  that  has  been  partially 
digested  by  the  proteolytic  enzymes  of  the  stomach  and  intes- 
tines. The  products  of  this  action  are  more  or  less  characteristic 
because  of  the  peculiar  manner  in  which  the  aromatic  groups  of 
the  protein  molecule  are  attacked,  producing  from  it  such  sub- 
stances as- phenol,  skatol,  indol,  etc.,  to  which  the  characteristic 
odor  of  the  faeces  is  due.  When  protein  has  been  adequately  di- 
gested in  the  stomach,  it  is  so  rapidly  acted  on  by  the  trypsin 
(and  erepsin)  of  the  small  gut  and  is  so  quickly  absorbed  that 
bacteria  have  no  chance  to  act  on  it.  When  protein  has  been  in- 
adequately digested  in  the  stomach,  however,  the  trypsin  fails  to 
digest  it  quickly  enough,  so  that  bacterial  putrefaction  sets  in 
which  may  be  quite  marked  in  the  small  intestine,  although  much 
more  so  in  the  colon.  Even  when  they  do  not  find  a  suitable  sub- 
strat  in  the  food,  the  bacteria  attack  the  proteins  of  the  intes- 
tinal secretions  themselves,  which  accounts  for  the  well-known 
occurrence  of  this  process  during  starvation. 

The  Immunity  of  the  Walls  of  the  Digestive  Organs  Toward 
the  Enzymes  Which  Act  within  Them. — The  immunity  of  the 
mucosa  of  the  stomach  and  intestines  seems  to  be  due  in  main  to 
the  presence  in  the  cells  of  the  mucosa  of  anti-enzymes,  that  is,  of 
substances  which  can  inhibit  the  action  of  the  various  enzymes 


78  HUMAN   PHYSIOLOGY. 

(antipepsin,  antitrypsin,  etc.).  As  we  should  expect,  very  strong 
anti-enzymes  can  be  prepared  from  tapeworms  and  other  intes-. 
tinal  worms.  It  is  by  virtue  of  possessing  these,  that  the  worms 
are  not  digested.  The  immunity  of  the  gland  cells  and  duets,  as 
of  the  pancreas,  to  the  proteolytic  enzymes  which  they  produce 
is  possibly  to  be  explained  in  another  way,  namely,  by  the  ex- 
istence of  the  enzyme  as  an  inactive  precursor  (e.  g.,  trypsino- 
gen)  until  after  the  secretion  has  been  carried  to  a  region  whose 
walls  contain  the  specific  anti-body.  A  certain  degree  of  im- 
munity to  the  destructive  action  of  the  intestinal  bacteria  on  the 
mucous  membrane  may  be  conferred  by  the  mucin,  which  is  quite 
abundant,  at  least  in  the  empty  stomach  and  in  the  large  intes- 
tine. The  relatively  poor  growth  of  bacteria  which  occurs  on 
inoculating  faecal  matter  in  culture  media — although  many  bac- 
teria can  be  seen  by  microscopic  examination  to  be  present — is 
probably  to  be  explained  by  their  having  been  killed  by  the 
mucin. 

The  Movements  of  the  Intestines. 

The  Movements  of  the  Small  Intestine  have  two  functions :  (1) 
to  macerate  and  mix  up  the  food  and  (2)  to  move  it  along  to- 
wards the  lower  end  of  the  gut.  These  two  functions  are  sub- 
served by  two  different  types  of  movement,  the  so-called  pendular 
and  the  peristaltic.  The  pendular  movements  are  rendered  evi- 
dent by  allowing  the  intestine  to  float  out  in  a  bath  of  isotonic 
saline  (p.  30),  when  the  various  loops  sway  from  side  to  side  like 
a  pendulum.  By  closer  examination  it  can  be  seen  that  the  move- 
ments are  produced  by  faint  waves  of  contraction  of  both  muscu- 
lar coats,  which  sweep  with  considerable  rapidity  along  the  gut. 
When  the  waves  arrive  at  a  part  of  the  intestine  containing  any 
solid  substance,  they  become  accentuated,  and  this  becomes  most 
marked  at  the  middle  of  the  solid  mass  of  food,  thus  tending,  on 
account  of  the  contraction  of  the  circular  fibers,  to  divide  the 
mass  into  two.  These  movements  are  therefore  somtimes  called 
segmenting  movements.  Their  function  is  evidently  to  break  up 
the  food  masses  and  thus  mix  the  food  with  the  digestive  juices. 
This  can  be  very  well  shown  in  skiagram  shadows  of  the  ab- 


INTESTINAL  DIGESTION.  79 

domen  some  time  after  taking  food  mixed  with  bismuth.  A 
column  of  food  can  be  seen  to  divide  into  several  segments,  each 
of  which  in  a  few  seconds  breaks  into  two,  the  neighboring 
halves  then  joining  together,  and  the  process  repeating  itself. 

Two  varieties  of  peristaltic  waves  are  usually  described,  both 
of  which  are  characterized  by  a  marked  constriction  preceded  by 
a  distinct  dilatation  of  the  gut,  which  may  extend  for  a  consid- 
erable distance  down  it  (two  feet).  The  one  variety  of  wave 
travels  slowly  (I/2  cm.  per  minute),  and  has  the  function  of  car- 
r3'ing  along  the  food ;  the  other  travels  very  rapidly  (peristaltic 
rush),  and  is  evidently  for  the  purpose  of  hurrying  along  irri- 
tating substances. 

Besides  being  set  up  t)y  the  presence  of  food  in  the  intestine, 
these  waves  may  be  influenced  through  the  nervous  system ;  stim- 
ulation of  the  vagus  excites  them,  whereas  stimulation  of  the 
sympathetic  brings  about  a  marked  inhibition,  in  which  the  whole 
gut  becomes  profoundly  relaxed  with  the  exception  of  the  ileo- 
colic sphincter,  which  contracts.  This  influence  of  the  splanch- 
nic may  be  excited  reflexly,  as  by  pain  or  fear. 

The  Movements  of  the  Large  Intestine  are  more  difficult  to 
study  than  those  of  the  small  intestine.  They  vary  considerably  in 
different  animals,  as  indeed  is  to  be  expected  when  we  remember 
that  the  function  of  this  part  of  the  alimentary  tract  depends 
upon  the  nature  of  the  food.  In  herbivora,  for  example,  food 
ma}'  lie  in  the  capacious  caecum  for  days,  and  even  in  carnivora, 
in  which  this  part  of  the  gut  is  rudimentary,  it  may  remain  for 
twenty-four  hours.  In  man  the  conditions  seem  to  be  intermedi- 
ate between  those  in  the  herbivora  and  carnivora,  and  the  move- 
ments are  believed  to  be  as  follows:  As  the  semi-fluid  food  en- 
ters the  caecum  through  the  ileo-caecal  valve  and  collects  in  the 
cfficum  and  proximal  colon,  it  excites  the  occurrence  of  waves  of 
constriction,  which  start  probably  about  the  hepatic  flexure  and 
travel  in  a  central  direction  towards  the  caecum,  into  which  the 
food  is  thus  forced  back. 

Occasionally  the  arrival  of  the  wave  at  the  caseum  starts  a 
true  i)eristaltic  wave,  which  travels  distally,  getting  feebler  as 
it  i)rofeeds,  and  which  may  carry  some  of  the  contents  into  the 


80 


HUMAN   PHYSIOLOGY, 


transverse  colon.  Here  the  contents  assume  more  or  less  of  the 
consistency  of  fgeces,  and  more  powei-fnl  peristaltic  waves  make 
their  appearances  so  that  the  solid  masses  are  carried  on  towards 
the  rectum.  These  waves  are  sufficiently  energetic  to  keep  the 
descending  colon  comparatively  empty,  and  the  feecal  masses 
gradually  accumulate  in  the  sigmoid  flexure  and  rectum  until 
evacuated  by  the  act  of  defsecation. 

Examination  of  the  accompanying  diagram  (Fig.  7)  will  show 
how  long  food  takes  to  pass  along  the  various  parts  of  the  gastro- 
intestinal tract. 


Fig-.    7. — Diagram   of   time    it    takes    for   a    capsule    containing   bismuth    to 
reacli  tlie  various  parts  of  thie  large  intestine. 


The  Absorption  of  Food. 

As  has  been  explained,  the  whole  object  of  digestion  is  to  break 
up  the  large  molecules  of  which  food  is  composed  into  smaller 
ones  so  that  they  can  be  absorbed  into  the  blood  or  lymph  which 
circulates  in  the  mucous  membrane  of  the  intestines.  Except  un- 
der unusual  circumstances,  no  absorption  occurs  until  the  small 
intestine  is  reached.     Here  sugars  are  absorbed  into  the  blood 


THE  ABSORPTION  OF  FOOD.  81 

as  dextrose,  and  proteins  as  amino  acids,  whilst  fats  are  ab- 
sorbed into  the  lymphatic  vessels,  as  fatty  acids  and  glycerine. 
These  substances  are  absorbed  in  solution,  which  would  lead  us 
to  expect  that,  because  of  the  water  absorbed  along  with  them, 
the  contents  of  the  small  intestine  would  be  more  solid  at  its 
lower  than  at  its  upper  end;  but  this  is  not  the  case,  for  the 
digestive  juices  which  have  been  secreted  make  up  for  the  loss 
of  water.  It  is  in  the  large  intestine  that  the  water  is  finally 
absorbed. 

Attempts  have  been  made  to  explain  the  mechanism  of  ab- 
sorption in  terms  of  the  known  laws  of  filtration,  osmosis,  surface 
tension,  and  imbibition,  but  little  further  progress  has  been 
made  than  to  establish  the  fact  that  although  these  processes 
may  play  a  role,  they  are  not  alone  responsible.  Thus,  if  blood 
serum  be  placed  in  an  isolated,  loop  of  intestine,  it  will  become 
entirely  absorbed,  even  although  identical  in  all  the  above  prop- 
erties with  the  blood  of  the  animal.  That  osmosis  does  have 
some  influence,  however,  is  evidenced  by  the  well-known  effect 
of  a  strong  saline  solution  in  the  intestine;  it  attracts  water 
from  the  blood,  thus  diluting  the  intestinal  contents  and  stim- 
ulating peristaltic  contractions.  It  is  in  this  way  that  saline 
cathartics  act. 

Regarding  the  absorption  of  fats,  it  is  now  definitely  known 
that  these  are  first  of  all  split  into  fatty  acid  and  glycerine  by 
the  action  of  the  lipase  of  pancreatic  juice.  The  fatty  acid  then 
unites  with  alkali  to  form  a  soap,  or  with  bile  salts  to  form  a  sol- 
uble compound.  In  either  case,  the  dissolved  fatty  acid  passes 
into  the  intestinal  epithelium,  into  which  is  also  absorbed  the 
glycerine,  the  two  re-uniting  after  their  absorption  so  as  to  form 
neutral  fat  again.  The  neutral  fat  then  passes  into  the  central 
lacteal  of  the  villus,  whence  it  is  transported  by  the  abdominal 
lymphatics  to  the  thoracic  duct,  which  discharges  it  into  the 
subclavian  vein  on  the  left  side  of  the  root  of  the  neck. 

Hunger  sensations  coincide  with  stomach  contractions,  but 
these  differ  from  those  which  occur  during  digestion.  Thirst  is 
due  to  dryness  of  the  throat.  It  is  temporarily  relieved  by 
moistening  the  throat,  but  unless  liquid  is  swallowed  permanent 
thirst  develops  because  the  tissues  become  dry. 


Resume  of  Actions  of  Digestive  Enzymes. 


Secretion 


Enzyme  ob 

Adjuvant 

Agency 


Saliva 


Gastric   juice.. 


Pancreatic 
juice  .   . . 


Bile 


Intestinal 
juice  .  . . 


Bacteria 


Ptyalin . . 
Alkalies 
Pepsin  . 


HCl. 


Lipase  

Trypsinogen 
Lipase  


Amylopsin 
Alkali  .  . . , 


Bile  salts 


Alkali 


Enterokinase. 


Action 


Erepsin  

Inverting 
enzymes  .  ... 


Acting  on 
carboliydrates 

Acting  on 
fats 

Acting  on 
proteins  .    . 


Converts  boiled  starch  into  maltose. 
Favors  action  of  ptyalin. 

(1)  Converts   metaproteins    (acid   albu- 

min, etc.)  into  proteoses  and  pep- 
tones. 

(2)  Clots  milk. 

(1)  Produces  metaproteins. 

(2)  Acts  as  antiseptic. 

(3)  Stops  action  of  ptyalin. 
Acts  on  emulsified  fats. 

Inactive  until  acted  on  by  enterokinase. 
Splits   neutral   fat   into   fatty   acid   and 

glycerine. 
Converts  all  starches  into  maltose. 

(1)  Helps  to  neutralize  HCl  of  chyme. 

(2)  Combines  with  fatty  acid  to  form 

soaps. 

(1)  Augment  the  action  of  lipase  and 

and  amylopsin. 

(2)  Precipitate  pepsin  and  peptones  in 

chyme. 

(3)  Combines  with  fatty  acids. 

(1)  Helps  to  neutralize  HCl  of  chyme. 

(2)  Combines  with  fatty  acid  to  form 

soaps. 

Converts      trypsinogen      into      trypsin, 

which  splits  proteins  into  amino 

bodies. 
Converts  caseinogen  and  peptones  into 

simple  amino  bodies. 
One     for    each    disaccharide,    splitting 

them  into  monosaccharides. 

(Both  the  last  two  enzymes  are  more 
plentiful  in  the  epithelium  than 
in  the  intestinal  juice.) 

(1)  Digest  cellulose. 

(2)  Splits   monosaccharides   into   lactic 

and  lower  acids. 

Split  higher,  into  lower  fatty  acids. 

Split  off  aromatic  groups,  as  phenol, 
cresol,  etc. 

(Besides  these  specific  actions,  bacteria 
may  perform  many  of  the  diges- 
tive functions  of  the  juices.) 


CHAPTER  VIII. 

METABOLISM. 

The  Energy  Balance. 

Introductory. — The  object  of  digestion,  as  we  have  seen,  is 
to  render  the  food  capable  of  absorption  into  the  circulatory 
fluids,  the  blood  and  lymph.  The  absorbed  food  products  are 
then  transported  to  the  various  organs  and  tissues  of  the  body, 
where  they  may  be  either  used  or  stored  away  against  future 
requirements.  After  being  used,  certain  substanc.es  are  produced 
as  waste  products,  and  these  pass  back  into  the  blood  to  be  car- 
ried to  the  organs  of  excretion,  by  which  they  are  expelled  from 
the  body.  By  comparison  of  the  amount  of  these  excretory  prod- 
ucts with  that  of  the  constituents  of  food,  we  can  tell  how  much 
of  the  latter  has  been  retained  in  the  body,  or  lost  from  it.  This 
constitutes  the  subject  of  general  metabolistn.  On  the  other 
hand,  we  may  direct  our  attention,  not  to  the  balance  between 
intake  and  output,  but  to  the  chemical  changes  through  which 
each  foodstuff  must  pass  between  its  absorption  and  excretion. 
This  is  the  subject  of  special  metaljolism.  In  the  one  case  we 
content  ourselves  with  a  comparison  of  the  raw  material  which 
is  acquired  and  the  finished  product  which  is  produced  by  the 
animal  factory;  in  the  other,  we  seek  to  learn  something  of  the 
particular  changes  to  which  each  crude  product  is  subjected  be- 
fore it  can  be  used  for  the  purpose  of  driving  the  machinery  of 
life  or  of  repairing  the  worn  out  parts  of  the  body. 

In  drawing  up  such  a  balance  sheet  of  general  metabolism,  we 
must  select  for  comparison  substances  which  are  common  to  both 
intake  and  output.  In  general  the  intake  comprises,  besides  oxy- 
gen, the  proteins,  fats  and  carbohydrates,  and  the  output,  carbon 
dioxide,  water  and  the  various  nitrogenous  constituents  of  urine. 
This  dissimilarity  in  chemical  structure  between  the  substances 
ingested  and  those  excreted    limits    us,    in    balancing    the    one 

83 


84  HUMAN  PHYSIOLOGY.  * 

against  the  other,  to  a  comparison  of  the  smallest  fragments 
into  which  each  can  be  broken.  Such  fragments  are  the  ele- 
ments, and  of  these  carbon  and  nitrogen  alone  can  be  measured 
with  accuracy  in  both  intake  and  output.  From  the  balance 
sheets  of  intake  and  output  of  carbon  and  nitrogen,  and  from 
information  obtained  by  observing  the  ratio  between  the 
amounts  of  oxygen  consumed  by  the  animal  and  of  carbonic  - 
acid  (CO J  excreted,  we  can  draw  far-reaching  conclusions 
regarding  the  relative  amounts  of  protein,  fat  and  carbohydrate 
which  have  participated  in  the  metabolism. 

As  has  already  been  stated,  the  essential  nature  of  the  meta- 
bolic process  in  animals  is  one  of  oxidation,  that  is,  one  by 
which  large  unstable  molecules  are  broken  down  to  those  that 
are  simple  and  stable.  During  this  process  of  hatabolism,  as 
it  is  called,  the  potential  energy  locked  away  in  the  large  mole- 
cules becomes  liberated  as  actual  or  kinetic  energy,  which  takes 
the  form  of  movement  and  heat.  It  therefore  becomes  of  im- 
portance to  compare  the  actual  energy  which  an  animal  ex- 
pends in  a  given  time  with  the  energy  which  has  meanwhile 
been  rendered  available  by  metabolism.  This  is  called  the 
energy  'balance.  We  shall  first  of  all  consider  this  and  then 
proceed  to  examine  somewhat  more  in  detail  the  material  bal- 
ance of  the  body. 

Energy  Balance. 

The  unit  of  energy  is  the  large  calorie  (written  C),  which  is 
the  amount  of  heat  required  to  raise  the  temperature  of  one  kilo-  3 
gramme  of  water  through  one  degree  (Centigrade)  of  tempera- 
ture.^ "We  can  determine  the  caloric  value  by  allowing  a  meas- 
ured quantity  of  a  substance  to  burn  in  compressed  oxygen  in 
a  steel  bomb  which  is  placed  in  a  known  volume  of  water  at  a 
certain  temperature.  Whenever  combustion  is  completed,  we 
ascertain  the  increase  in  temperature  of  the  water  in  degrees 
(Centigrade),   and   mulitply  this  by  the   volume   of  water  in 


iThe  distinction  between  a  calorie  and  a  degree  of  temperature  must  be 
clearly  understood.  The  former  expresses  quantity  of  actual  heat  energy ; 
the  latter  merely  tells  us  the  intensity  at  which  the  heat  energy  is  being  given 
out. 


THE  ENERGY   BALANCE.  85 

liters.     Measured  in  such  a  calorimeter,  as  this  apparatus  is 

called,  it  has  been  found  that  the  number  of  calories  liberated 

by  burning  one  gramme  of  each  of  the  proximate  principles  of 

food  is  as  follows : 

^     ,    ,     ,     ,       (Starch    4.1 

Carbohydrates  I  g^g^^ 40 

Protein    5.0 

Fat    9.3 

The  same  number  of  calories  will  be  liberated  at  whatever  rate 
the  combustion  proceeds,  provided  it  results  in  the  same  end 
products.  Wlien  a  substance,  such  as  sugar  or  fat,  is  burned  in 
the  presence  of  oxygen,  it  yields  carbon  dioxide  and  water,  which 
are  also  the  end  products  of  the  metabolism  of  these  foodstuffs 
in  the  animal  body;  therefore,  when  a  gramme  of  sugar  or  fat 
is  rapidly  burned  in  a  calorimeter,  it  releases  the  same  amount 
of  energy  as  when  it  is  slowly  oxidized  in  the  animal  body.  But 
the  case  is  different  for  proteins,  because  these  yield  less  com- 
pletely oxidized  end-products  in  the  animal  body  than  they  yield 
when  burned  in  oxygen;  so  that,  to  ascertain  the  physiological 
energy-value  of  protein,  we  must  deduct  from  its  physical  heat 
value  (calories)  the  physical  heat-value  of  the  incompletely  ox- 
idized end-products  of  its  metabolism.  It  is  obvious  that  we  can 
compute  the  total  available  energy  of  our  diet  by  multiplying 
the  quantity  of  each  foodstuff  by  its  caloric  value. 

In  order  to  measure  the  energy  which  is  actually  liberated  in 
the  animal  body,  we  must  also  use  a  calorimeter,  but  of  some- 
what different  construction  from  that  used  by  the  chemist,  for 
we  have  to  provide  for  long  continued  observations  and  for  an 
uninterrupted  supply  of  oxygen  to  the  animal.  Anwial  calor- 
imeters are  also  usually  provided  with  means  for  the  measure- 
ment of  the  amounts  of  carbon  dioxide  (and  water)  discharged 
and  of  oxygen  absorbed  by  the  animal  during  the  observation. 
Such  respiration  calorimeters  have  been  made  for  all  sorts  of  ani- 
mals, the  most  perfect  for  use  on  man  having  been  constructed 
in  America  (see  Fig.  8).  As  illustrating  the  extreme  accuracy 
of  even  the  largest  of  these,  it  is  interesting  to  note  that  the  act- 
ual heat  given  out  when  a  definite  amount  of  alcohol  or  ether  is 


86 


HUMAN  PHYSIOLOGY. 


burned  in  one  of  them  exactly  corresponds  to  the  amomit  as  meas- 
ured by  the  smaller  bomb  calorimeter.  All  of  the  energy  liber- 
ated in  the  body  does  not,  however,  take  the  form  of  heat.  A 
variable  amount  appears  as  mechanical  work,  so  that  to  measure 
in  calories  all  of  the  energy  which  an  animal  expends,  one  must 
add  to  the  actual  calories  given  out,  the  caloric  equivalent  of 

T 


<()   *       WOitier  to  absorb    h<z(A 


Fig.  8. — Diagram  of  Atwater-Benedict  Respiration  Calorimeter.  As  the 
animal  uses  up  the  Oj,  the  total  volume  of  air  shrinks.  This  shrinkage  is  indi- 
cated by  the  meter,  and  a  corresponding  amount  of  O2  is  delivered  from  the 
weighed  Oa-cylinder.  The  increase  in  weight  of  bottles  II  and  III  gives  the 
CO2. 

the  muscular  work  which  has  been  performed  by  the  animal 
during  the  period  of  observation.  This  can  be  measured  by 
means  of  an  ergometer,  a  calorie  corresponding  to  425  kilo- 
grammeters-  of  work.  That  it  has  been  possible  to  strike  an 
accurate  balance  between  the  intake  and  the  output  of  energy 
of  the  animal  body,  is  one  of  the  achievements  of  modern  experi- 
mental biology.     It  can  be  done  in  the  case  of  the  human  ani- 


2  A  kilogrammeter  is   the  product  of  the   load   in  kilograms   multiplied   by 
the  distance  in  meters  through  which  it  is  lifted. 


THE  ENERGY  BALANCE.  87 

mal ;  thus,  a  man  doing  work  on  a  bicycle  ergometer  in  the  Bene- 
dict calorimeter  gave  out  as  actual  heat,  4,833  C,  and  did  work 
equalling  602  C,  giving  a  total  of  5,435  C.  By  drawing  up 
a  balance  sheet  of  his  intake  and  output  of  food  material  during 
this  period,  it  was  found  that  the  man  had  consumed  an  amount 
capable  of  yielding  5,459  C,  which  may  be  considered  as  ex- 
actly balancing  the  actual  output. 

Having  thus  satisfied  ourselves  as  to  the  extreme  accuracy  of 
the  method  for  measuring  energy  output,  we  shall  now  consider 
some  of  the  conditions  which  control  it.  To  study  these  we  must 
first  of  all  determine  the  hasal  heat  production,  that  is,  the  small- 
est energy  output  which  is  compatible  with  health.  This  is  as- 
certained by  allowing  the  man  to  sleep  in  the  calorimeter  and 
then  measuring  his  calorie  output  while  he  is  still  resting  in  bed 
in  the  morning,  and  fifteen  hours  after  the  last  meal.  When 
the  results  thus  obtained  on  a  number  of  individuals  are  calcu- 
lated so  as  to  represent  the  calorie  output  per  kilogram  of 
body  weight  in  each  case,  it  will  be  found  that  1  C.  per  kilo  per 
hour  is  discharged.  That  is  to  say,  the  total  energy  expenditure 
in  24  hours  in  a  man  of  70  kilos,  w^hich  is  a  good  average  weight, 
will  be  70  X  24  =  1,680  C. 

When  food  is  taken  the  heat  production  rises,  the  increase  over 
the  basal  heat  production  amounting,  for  an  ordinary  diet,  to 
about  ten  per  cent.  Besides  being  the  ultimate  source  of  all  the 
body  heat,  food  is  therefore  a  direct  stimulant  of  heat  production. 
This  specific  dynamic  action,  as  it  is  called,  is  not,  however,  the 
same  for  all  groups  of  foodstuffs,  being  greatest  for  proteins  and 
least  for  carbohydrates.  Thus,  if  a  starving  animal  is  given  an 
amount  of  protein  which  is  equal  in  caloric  value  to  the  calorie 
output  during  starvation,  the  calorie  output  will  increase  by  30 
per  cent,  whereas  with  carbohydrates  it  will  increase  only  by 
6  per  cent.  Evidently,  then,  protein  liberates  much  free  heat 
during  its  assimilation  in  the  minimal  body ;  it  burns  with  a  hot- 
ter flame  than  fats  or  carbohydrates,  although,  as  in  the  case 
of  fats,  at  least,  before  it  is  completely  burnt,  it  may  not  yield  so 
much  energy.  This  peculiar  property  of  proteins  accounts  for 
their  well-known  heating  qualities.    It  explains  why  protein  com- 


88  HUMAN   PHYSIOLOGY. 

poses  SO  large  a  proportion  of  the  diet  of  peoples  living  in  cold 
regions,  and  why  it  is  cut  down  in  the  diet  of  those  who  dwell 
near  the  tropics.  Individuals  maintained  on  a  low  protein  diet 
may  suffer  intensely  from  the  cold. 

If  we  add  to  the  basal  heat  production  of  1,680  C.  another 
168  C.  (or  10  per  cent)  on  account  of  food,  the  total  1,848  C. 
nevertheless  falls  far  short  of  that  which  we  know  must  be  liber- 
ated when  we  calculate  the  available  energy  of  the  diet.  What 
becomes  of  the  extra  fuel?  The  answer  is  that  it  is  used  for, 
muscular  work.  Thus  it  has  been  found  that  if  the  observed 
person,  instead  of  lying  down  in  the  calorimeter,  is  made  to  sit 
in  a  chair,  the  heat  production  is  raised  by  8  per  cent,  or  if 
he  performs  such  movements  as  would  be  necessary  for  ordinary 
work  (writing  at  a  desk) ,  it  may  rise  29  per  cent,  that  is  to  say, 
to  90  C.  per  hour.  Allowing  8  hours  for  sleep  and  16  hours  for 
work,  we  can  thus  account  for  2,168  C,  the  remaining  300  odd 
C.  which  is  required  to  bring  the  total  to  that  which  we  know, 
from  statistical  tables  of  the  diets  of  such  workers,  to  be  the 
actual  daily  expenditure,  being  due  to  the  exercise  of  walking. 
If  the  exercise  be  more  strenuous,  still  more  calories  will  be  ex- 
pended; thus,  to  ascend  a  hill  of  1,650  feet  at  the  rate  of  2.7 
miles  an  hour  requires  407  extra  calories.  Field  workers  may 
expend,  in  24  hours,  almost  twice  as  many  calories  as  those  en- 
gaged in  sedentary  occupations. 

'  Another  factor  which  controls  the  energy  output  i»  the  cool- 
ing  influence  of  the  atmosphere.  When  this  is  marked,  more 
heat  must  be  liberated  in  order  to  maintain  the  body  temperature 
(see  p.  135).  In  other  words,  the  necessary  heat  loss  must  be 
compensated  by  an  increased  heat  production,  just  as  we  must 
burn  more  coal  to  keep  the  house  at  a  given  temperature  on  a 
cold,  than  on  a  warm,  day.  This  adjustment  of  energy  liberation 
to  the  rate  of  cooling  at  the  surface  of  the  body  explains,  among 
other  things,  why  it  should  be  that  small  animals  give  out  much 
more  energy,  per  unit  of  body  weight,  than  those  that  are  larger. 
The  small  animal  has  relatively  the  greater  surface  area,  just 
as  two  cubes  of  equal  weight  when  brought  together  have  a  com- 
bined weight  which  is  double  that  of  either  cube,  but  a  surface 


THE   ENERGY   BALANCE. 


89 


area  which  is  less<than  double  (two  surfaces  having  been  brought 
together).  Greater  tendency  to  surface  cooling  explains  why 
small  animals  should  so  much  more  quickly  succumb  to  cold 
than  those  that  are  larger,  and  why  slim  persons  should  feel 
the  cold  more  keenly  than  those  that  are  stout. 

Other  things,  such  as  diet,  external  temperature,  etc.,  being 
the  same,  it  is  therefore  surface  area  and  not  tody  weight  which 
determines  the  energy  production,  a  fact  which  is  clearly  dem- 
onstrated by  finding  that  the  calorie  output  for  different  animals 
is  constant  when  it  is  calculated  for  each  square  meter  of  sur- 
face. Thus,  a  horse  produces  only  14.5  C.  per  kg.  of  body 
weight  in  24  hours,  whereas  a  mouse  produces  452  C,  but  if 
we  calculate  according  to  square  meter  of  surface  the  dif- 
ferences practically  vanish.  These  facts,  however,  do  not  apply 
when  the  differences  in  size  are  due  to  age.  This  has  been 
most  strikingly  demonstrated  in  the  case  of  man,  for  it  has  been 
found  that  the  calorie  requirement  per  unit  of  surface  is  very 
distinctly  greater  in  the  early  years  of  life  than  later.  Thus,  tak- 
ing the  discharge  of  carbon  dioxide  as  a  criterion  of  the  energy 
discharge,  the  following  results  have  been  obtained  from  indi- 
viduals sitting  down : 


Carbon   dioxide   discharged,   per 

verage  age 

Average  weigjit 

square  meter  of  surface 

(years) 

(kilograms) 
Males 

and  hour    (grams) 

9  2/3 

28 

29.9 

12   1/2 

34 

26.5 

15  1/2 

51 

23.5 

19  1/2 

60 

21.8 

25 

68 

18.5 

35 

68 

16.9 

45 

77 

16.3 

58 

85 
Females 

14.2 

S 

22 

26.6 

12 

36 

20.1 

15 

49 

16.0 

17  2/3 

54 

14.8 

30 

54 

16.3 

45 

67 

17.9 

90  HUMAN   PHYSIOLOGY. 

This  table  shows  us  clearly  that  over  and  above  the  greater 
combustion  necessary  on  account  of  their  relatively  greater  sur- 
face, children  require  calories  for  growth.  They  must  be  fed 
more  liberally  than  adults,  otherwise  they  starve.  The  table 
further  shows  that  boys  must  be  more  liberally  fed  than  girls  of 
equal  age  and  body  weight,  probably  because  of  their  greater 
restlessness.  It  is  on  account  of  these  greater  food  requirements 
that  children  are  the  first  to  die  in  famine. 

Recent  work  has  shown  that  the  above  conclusions  are  not 
strictly  warranted  by  the  facts,  for  there  appear  to  be  other 
factors  than  surface  and  mass  of  the  body  affecting  the  energy 
requirement  of  the  growing  organism. 


CHAPTER  IX. 

METABOLISM  (Cont'd). 

The  Material  Balance  of  the  Body. 

We  must  distinguish  between  the  balances  of  the  organic  and 
the  inorganic  foodstuffs.  From  a  study  of  the  former  we  shall 
gain  information  regarding  the  sources  of  the  energy  production 
whose  behavior  under  various  conditions  we  have  just  studied. 
From  a  study  of  the  inorganic  balance,  although  we  shall  learn 
nothing  regarding  energy  exchange — for  such  substances  can 
yield  no  energy — we  shall  become  acquainted  with  several  facts 
of  extreme  importance  in  the  maintenance  of  nutrition  and 
growth. 

To  draw  up  a  lyalance  sheet  of  organic  intake  and  output  re- 
quires an  accurate  chemical  analysis  of  the  food  and  of  the 
excreta  (urine  and  expired  air).  Furnished  with  such  analyses 
we  proceed  to  ascertain  the  total  amount  of  nitrogen  and  carbon 
in  the  excreta  in  a  given  time  and  to  calculate,  from  the  known 
percentage  of  nitrogen  in  protein,  how  much  protein  must  have 
undergone  metabolism.  We  then  compute  how  much  carbon  this 
quantity  of  protein  would  account  for,  and  we  deduct  this  from 
the  total  carbon  excretion.  The  remainder  of  carbon  must  have 
come  from  the  metabolism  of  fats  and  carbohydrates,  and  al- 
though we  cannot  tell  exactly  its  source,  yet  we  can  arrive  at  a 
close  approximation  by  observing  the  respiratory  quotient  (R. 
Q.),  which  is  the  ratio  of  the  volume  of  carbon  dioxide  exhaled 

CO, 
to  that  of  oxygen  retained  by  the  body  in  a  given  time,  i.  e., 

0, 
When  carbohydrates  are  the  only  foodstuff  undergoing  metabol- 
ism, the  quotient  is  one,  that  is  to  say,  the  CO2  excretion  and  0, 
intake  are  equal  in  volume.  The  reason  for  this  is  that  a  molecule 
of  carbohydrate  consists  of  C  along  with  PI  and  0  in  the  same 
proportions  as  they  exist  in  water ;  therefore  oxygen  is  required 

91 


92  HUMAN  PHYSIOLOGY. 

to  oxidize  tlie  C,  but  not  the  H.,  and,  since  equimoleeular  quan- 
tities of  all  gases  occupy  equal  volumes  (at  the  same  tempera- 
ture and  pressure),  the  volume  of  COo  produced  equals  the  vol- 
ume of  C.  required  to  produce  it.  The  conditions  are  other- 
wise in  the  case  of  fats  and  proteins,  for  besides  C.  these  mole- 
cules contain  an  excess  of  H.,  so  that  O.  is  required  to  oxidize 
some  of  the  H.,  as  well  as  all  of  the  C.  A  greater  volume  of  Oo 
is  therefore  absorbed  during  their  combustion  than  the  volume 
of  CO2  that  is  produced,  and  R.  Q.  is  about  0.7.  By  observing 
this  quotient,  therefore,  we  can  approximately  determine  the 
source  from  which  the  non-protein  carbon  excretion  is  derived. 
Having  in  the  above  manner  computed  how  much  of  each  of  the 
proximate  principles  has  undergone  metabolism,  we  next  pro- 
ceed to  compare  intake  and  output  with  a  view  to  finding 
whether  there  is  an  equilibrium  between  the  two,  or  whether  re- 
tention or  loss  is  occurring. 

Starvation. — In  order  to  furnish  us  with  a  standard  condition 
with  which  we  may  compare  others,  we  will  first  of  all  study  the 
metabolism  during  starvation.  When  an  animal  is  starved,  it 
has  to  live  on  its  own  tissues,  but  in  doing  so,  it  saves  its  protein 
so  that  the  excretion  of  nitrogen  falls  after  a  few  days  to  a  low 
level,  the  energy  requirements  being  meanwhile  supplied,  as 
much  as  possible,  from  stored  carbohydrate  and  fat.  Although 
always  small  in  comparison  with  fat,  the  stores  of  carbohydrate 
vary  considerably  in  different  animals.  They  are  much  larger 
in  man  and  the  herbivora  than  in  the  carnivora.  During  the 
first  feiv  days  of  starvation  it  is  common,  in  the  herbivora,  to 
find  that  the  excretion  of  nitrogen  is  actually  greater  than  it  was 
before  starvation,  because  the  custom  has^  become  established  in 
the  metabolism,  of  these  animals  of  using  carbohydrates  as  the 
main  fuel  material,  so  that  when  this  fuel  is  withheld,  as  in 
starvation,  proteins  are  used  more  than  before  and  the  nitrogen 
excretion  becomes  greater.  We  may  say  that  the  herbivorous 
animal  has  become  carnivorous.  The  same  thing  may  occur  in 
man  when  the  previous  diet  was  largely  carbohydrate. 

During  the  greater  part  of  starvation,  however,  most  of  the 
energy  required  to  maintain  life  is  derived  from  fat,  as  little 


STARVATION.  yd 

as  possible  being  derived  from  protein.  This  type  of  metabolism 
lasts  until  all  the  available  resources  of  fat  have  become  ex- 
hausted, when  a  more  extensive  metabolism  of  protein  sets  in 
with  the  consequence  that  the  nitrogen  excretion  rises.  This  is 
really  the  harbinger  of  death — it  is  often  called  the  premortal 
rise  in  nitrogen  excretion.  It  means  that  all  the  ordinary  fuel 
of  the  animal  economy  has  been  used  up,  and  that  it  has  become 
necessary  to  burn  the  very  tissues  themselves  in  order  to  obtain 
sufficient  energy  to  maintain  life.  Working  capital  being  all 
exhausted,  an  attempt  is  made  to  keep  things  going  for  a  little 
longer  time  by  liquidation  of  permanent  assets.  But  these  assets, 
as  represented  by  protein,  are  of  little  real  value  in  yielding  the 
desired  energy  because,  as  we  have  seen,  only  4.1  calories  are 
available  against  9.3,  obtainable  from  fats.  These  facts  explain 
why  during  starvation  a  fat  man  excretes  daily  less  nitrogen 
than  a  lean  man,  and  why  the  fat  man  can  stand  the  stai-vation 
for  a  longer  time. 

Not  only  is  there  this  general  saving  of  protein  during  star- 
vation, but  there  is  also  a  discriminate  utilization  of  what  has 
to  be  used  by  the  different  organs  according  to  their  relative 
activities.  This  is  very  clearly  shown  by  comparison  of  the  loss 
of  weight  which  each  organ  undergoes  during  starvation.  The 
heart  and  brain,  which  must  be  active  if  life  is  to  be  maintained, 
lose  only  about  3  per  cent  of  their  original  weight,  whereas  the 
voluntary  muscles,  the  liver  and  the  spleen  lose  31,  54  and  67 
per  cent,  respectively.  No  doubt  some  of  this  loss  is  to  be  ac- 
counted for  as  due  to  the  disappearance  of  fat,  but  a  sufficient 
remainder  represents  protein  to  make  it  plain  that  there  must 
have  been  a  mobilization  of  this  substance  from  tissues  where  it 
was  not  absolutely  necessary,  sujeh  as  the  liver  and  voluntary 
muscles,  to  organs,  such  as  the  heart,  in  which  energy  transfor- 
mation is  sine  qua  nan  of  life.  The  vital  organs  live  at  the  ex- 
pense of  those  whose  functions  are  accessory. 

When  we  compare  the  excretion  of  carbon  dioxide  from  day 
to  day  during  starvation,  it  will  be  found  to  remain  practically 
constant,  when  calculated  for  each  kilogram  of  body  weight.  The 
same  is  true  for  the  calorie  output.     Certain  unusual  substances 


94  HUMAN  PHYSIOLOGY., 

such  as  creatin  also  make  their  appearance  in  the  urine,  and 
there  is  an  increase  in  the  excretion  of  ammonia,  indicating  that 
larger  quantities  of  free  acid  are  being  set  free  in  the  organism. 

Starvation  ends  in  death  in  an  adult  man  in  somewhat  over 
four  weeks,  but  much  sooner  in  children,  because  of  their  more 
active  metabolism.  At  the  time  of  death  the  body  weight  may 
be  reduced  by  50  per  ceiit.  The  body  temperature  does  not 
change  until  within  a  few  days  of  death,  when  it  begins  to  fall, 
and  it  is  undoubtedly  true  that  if  means  be  taken  to  prevent  cool- 
ing of  the  animal  at  this  stage,  life  will  be  prolonged. 

Normal  Metabolism. — Apart  from  the  practical  importance 
of  knowing  something  about  the  behavior  of  an  animal  during 
starvation,  such  knowledge  is  of  great  value  since  it  furnishes  a 
standard  with  which  to  compare  tJie  metaholism  of  animals  under 
normal  conditions.  Taking  again  the  nitrogen  balance  as  indi- 
cating the  extent  of  protein  wear  and  tear  in  the  body,  let  us 
consider  first  of  all  the  conditions  under  which  equilibrium  may 
be  regained.  It  would  be  quite  natural  to  suppose  that  if  an 
amount  of  protein  containing  the  same  amount  of  nitrogen  as 
is  excreted  during  starvation  were  given  to  a  starving  animal, 
the  intake  and  output  of  nitrogen  would  balance.  We  are  led 
to  make  this  assumption  because  we  know  that  any  business  bal- 
ance sheet  showing  an  excess  of  expenditure  over  income  could 
be  met  by  such  an  adjustment.  But  it  is  a  very  different  matter 
with  the  nitrogen  halance  sJieet  of  the  body;  for,  if  we  give  the 
starving  animal  just  enough  protein  to  cover  the  nitrogen  loss, 
we  shall  eause  the  excretion  to  rise  to  a  total  which  is  practically 
equal  to  the  starvation  amount  plus  all  that  we  have  given  as 
food,  and  although  by  daily  giving  this  amount  of  protein  there 
may  be  a  slight  decline  in  the  excretion,  it  will  never  come  near 
to  being  the  same  as  that  of  the  intake.  Such  feeding  will'pro- 
long  life  for  a  few  days  only. 

To  strike  equilibrium  we  must  give  an  amount  of  protein  whose 
nitrogen  content  is  at  least  two  and  one-half  times  that  of  the 
starvation  level.  For  a  few  days  following  the  establishment  of 
this  more  liberal  diet,  the  nitrogen  excretion  will  be  far  in  ex- 
cess of  the  income,  but  it  will  gradually  decline  until  it  corre- 


NORMAL   METABOLISM.  95 

sponds  to  the  intake.  Having  once  gained  an  equilibrium,  we 
may  raise  its  level  by  gradually  increasing  the  protein  intake. 
During  this  progressive  raising  of  the  protein  intake,  it  will  be 
found,  at  least  in  the  carnivora  (cat  and  dog),  that  for  a  day 
or  so  immediately  following  each  increase  in  protein  intake, 
a  certain  amount  of  nitrogen  is  retained  by  the  body.  The  ex- 
cretion of  nitrogen,  in  other  words,  does  not  immediately  be- 
come adjusted  so  as  to  correspond  to  the  intake.  The  amount 
of  nitrogen  thus  retained  is  too  great  to  be  accounted  as,  a  re- 
tention of  disintegration  products  of  protein ;  it  must  there- 
fore be  due  to  an  actual  building  up  of  new  protein  tissue,  that 
is,  growth  of  muscles. 

Such  results  undoubtedly  obtain  in  the  cat,  and  less  mark- 
edly in  the  dog.  In  man  and  the  herbivorous  animals,  this  is 
not  the  case,  for  in  these  we  can  never  give  a  sufficiency  of  pro- 
tein alone  to  maintain  nitrogen  equilibrium;  there  will  always 
be  an  excess  of  excretion  over  intake.  But  indeed  it  scarcely  re- 
quires any'  experiment  to  prove  this,  for  it  is  self-evident  when 
we  consider  that  there  are  only  400  C.  in  a  pound  of  lean  meat, 
and  there  are  few  who  could  eat  more  than  4  pounds  a  day,  an 
amount  which  however  would  only  furnish  about  half  of  the  re- 
quired calories.  A  person  fed  exclusively  on  flesh  is  therefore 
being  partly  starved,  although  he  may  think  that  he  is  eating 
abundantly  and  be  quite  comfortable  and  active.  This  fact  has 
a  practical  application  in  the  so-called  Banting  cure  for  obesity, 
which  consists  essentially  in  limiting  the  diet  to  flesh  and  green 
vegetables,  allowing  only  a  very  small  quota  of  carbohydrates 
or  fats. 

Protein  Sparers. — Very  different  results  are  obtained  when 
carbohydrates  or  fats  are  freely  given  with  the  protein.  Nitrogen 
equilibrium  can  then  be  regained  on  very  much  less  protein ;  so 
we  speak  of  fats  and  carbohydrates  as  being  "protein  sparers." 
Carbohydrates  are  much  better  protein  sparers  than  fats ;  indeed 
they  are  so  efficient  in  this  regard  that  it  is  now  believed  that 
carbohydrates  are  essential  for  life,  so  that  when  the  food  con- 
tains no  carbohydrates,  a  part  of  the  carbon  of  protein  is 
converted  into  this  substance.  This  important  truth  is  supported 
by  evidence  derived  from  other  fields  of  investigation  (e.  g.,  the 


96  HUMAN  PHYSIOLOGY. 

behavior  of  diabetic  patients,  where  the  power  to  use  carbohy- 
drates is  much  depressed).  The  marked  protein-sparing  action 
of  carbohydrates  is  illustrated  in  another  way,  namely,  by  the 
fact  that  we  can  greatly  diminish  the  protein  break-down  during 
starvation  by  giving  carbohydrates.  In  this  way  we  can  indeed 
reduce  the  daily  nitrogen  excretion  to  about  one-third  what  it  is 
in  complete  starvation. 

The  Protein  Minimum. — In  the  case  of  man  living  on  an 
average  diet,  although  the  daily  nitrogen  excretion  is  about  15 
grams,  it  can  be  lowered  to  about  6  grams,  provided  that, 
in  place  of  the  protein  that  has  been  removed  from  the  diet, 
enough  carbohydrate  is  given  to  bring  the  total  calories 
up  to  the  normal  daily  requirement.  If  an  excess  of 
carbohydrate  over  these  energy  requirements  be  given,  the 
protein  may  be  still  further  reduced  and  yet  equilibrium  main- 
tained. To  do  this,  however,  it  is  not  the  amount  of  carbohy- 
drate alone  that  determines  the  ease  with  which  the  irreducible 
protein  minimicm  can  be  reached ;  the  kind  of  protein  itself  makes 
a  very  great  difference.  This  has  been  very  beautifully  shown 
by  one  investigator,  who  first  of  all,  determined  his  nitrogen  ex- 
cretion while  living  on  nothing  but  starch  and  sugar,  and  then 
proceeded  to  see  how  little  of  differnt  kinds  of  protein  he  had 
to  take  in  order  to  bring  himself  into  nitrogenous  equilibrium. 
He  found  that  he  had  to  take  the  following  amounts :  30  gm.  meat 
protein,  31  gm.  milk  protein,  34  gm.  rice  protein,  38  gm.  potato 
protein,  54  gm.  bean  protein,  76  gm.  bread  protein,  and  102  gm. 
Indian  corn  protein.  The  organism  is  evidently  able  to  satisfy 
its  protein  demands  when  it  takes  meat  protein  much  more 
readily  than  with  vegetable  proteins. 

To  understand  why  proteins  sliould  vary  so  much  in  their 
nutritive  value,  we  must  examine  their  ultimate  structure  very 
closely.  "When  the  protein  molecule  is  disintegrated,  as  by  diges- 
tion, it  yields  a  great  number  of  nitrogen-containing  acids,  the 
amino  acids,  as  well  as  several  bases  and  aromatic  substances. 
The  most  important  of  these  acids  are  glycin,  alanin,  serin,  valin, 
leucin,  prolin,  aspartic  and  glutamic  acids,  the  bases  being  lysin, 
histidin  and  arginin  and  the  aromatic  bodies,  phenylalanin,  tyro- 
sin  and  tryptophan.     These  substances  constitute  the  available 


NORMAL    METABOLISM.  97 

"units"  or  "building  stones"  of  protein  molecules,  but  in  no 
two  proteins  are  the  materials  used  exactly  in  the  same  propor- 
tions, some  proteins  having  a  preponderance  of  one  or  more  and 
an  absence  of  others,  just  as  in  a  row  of  houses  there  may  be  no 
two  that  are  exactly  alike,  although  for  all  of  them  the  same 
building  materials  Avere  available.  Albumin  and  globulin  are 
the  most  important  proteins  of  blood  and  tissues,  so  that  the 
food  must  contain  the  necessary  units  for  their  construction.  If 
it  fails  in  this  regard,  even  to  the  extent  of  lacking  only  one  of 
them,  the  organism  will  either  be  unable  to  construct  that  pro- 
tein, and  will  therefore  suffer  from  partial  starvation,  or  it  will 
have  to  construct  for  itself  this  missing  unit,  a  process  which  it 
can  accomplish  for  some  but  not  all  of  the  units. 

It  is  therefore  apparent  that  those  proteins  are  most  valu- 
able as  foods  that  contain  an  array  of  units  which  can  be  reunited 
to  form  all  the  varieties  of  protein  entering  into  the  structure 
of  the  body  proteins.  Naturally,  the  protein  which  most  nearly 
meets  the  requirement  is  meat  protein,  so  that  we  are  not  sur- 
prised to  find  that  less  of  it  than  of  any  other  protein  has  to  be 
taken  to  gain  nitrogen  equilibrium.  Casein,  the  protein  of  milk, 
although  it  does  not  contain  one  of  the  most  important  units, 
namely,  glycin,  is  almost  as  good  as  meat  protein,  because  the 
organism  is  itself  able  to  manufacture  glycin.  When,  on  the 
contrary,  proteins  (such  as  zein  from  corn)  are  given,  in  which 
certain  units  are  missing,  starvation  inevitably  ensues.  But  it 
does  not  do  so  if  the  missing  units,  (which  in  the  case  of  zein  is 
tryptophan)  are  added  to  the  diet. 

These  most  important  facts  have  been  ascertained  by  experi- 
ments carried  out  in  New  Haven  by  Osborne  and  Mendel. 
Young  albino  rats,  just  weaned,  were  fed  on  a  basal  diet  con- 
sisting of  the  sugar,  fat  and  salts  of  milk  to  which  was  added 
the  protein  whose  nutrition  value  it  was  desired  to  study.  The 
rats  were  weighed  from  day  to  day,  and  the  results  plotted 
as  a  curve — the  curve  of  growth.  A  gradually  rising  curve 
was  obtained  when  casein  or  the  albumin  of  milk  or  eggs,  or 
the  edestin  of  hemp  seed,  or  the  glutenin  of  wheat  was  fed, 
but  this  was  not  the  case  with  the  gliadin  of  wheat  or,   as 


98  HUMAX   PHYSIOLOGY. 

above  mentioned,  with  zein  of  corn.  It  will  be  seen,  there- 
fore, that  of  the  two  proteins  in  wheat  one,  glntenin,  contains 
all  the  necessary  units  for  building  up  the  growing  tissues,  but 
that  in  the  other  protein,  gliadin,  some  essential  unit  is  absent ; 
bv  analysis  this  was  found  to  be  lysin.  By  adding  Ij^sin  to 
gliadin  a  normal  curve  of  growth  resulted,  thus  showing  that 
this  was  really  the  missing  unit.  The  result  was  made  even  more 
spectacular  by  feeding  a  batch  of  young  rats  on  gliadin  alone, 
so  that  they  remained  undeveloped  and  stunted,  and  then  adding 
lysin  to  their  diet,  when  they  very  quickly  made  up  for  lost  time, 
and  soon  reached,  if  not  (]uite,  yet  almost  as  good  a  development 
as  their  more  fortunate  brothers  who  had  been  fed  on  glutenin 
or  casein  from  the  first. 

The  animal  economy  itself  can  therefore  produce  certain  of 
the  amino  bodies — thus,  as  we  have  seen,  it  can  produce  giycin — - 
this  power  being  much  more  developed,  in  the  case  of  herbivor- 
ous, as  compared  with  carnivorous  animals.  In  the  vegetable 
food  on  which  oxen  live,  several  of  the  prominent  amino  bodies 
of  muscle  protein  are  missing,  but  they  are  constructed  in  the 
organism  by  altering  the  arrangement  of  the  molecules  of  those 
amino  bodies  which  are  present,  so  that  a  protein  is  built  up 
which  is  very  like  that  present  in  the  tissue  of  the  carnivorous 
animals.  Even  in  the  case  of  the  herbivora.  however,  there  are 
limitations  to  the  power  of  forming  new  amino  bodies.  Trypto- 
phan, for  example,  cannot  be  formed  in  this  way. 


CHAPTER  X. 
THE  SCIENCE  OF  DIETETICvS. 

In  order  that  a  proper  assortment  of  amino  bodies  may  be 
assured  in  the  diet,  protein  is  taken  in  excess  of  the  quan- 
tity necessary  to  repair  the  tissues.  It  has  been  thought 
by  some  that  the  surplus  thus  taken  by  the  average  indi- 
vidual is  much  more  than  need  be,  and  that  an  unnecessary  strain 
is  thus  thrown  on  the  organs  which  have  to  dispose  of  the  excess. 
It  has  been  claimed  by  the  adherents  of  this  view  that  many  of 
the  obscure  symptoms — headaches,  muscular  and  back  pains, 
sleepiness,  etc.— that  city  folk  are  liable  to  suffer  from,  are  due 
to  the  presence  in  the  blood  of  unnecessary  by-products  of  ex- 
cessive protein  metabolism.  Such  opinions  seemed  to  receive 
very  weighty  indorsement  some  years  ago  when  Chittenden  pub- 
lished a  long  series  of  observations  showing  that  men  in  various 
callings  in  life,  could  perform  their  daily  work  quite  satisfac- 
torily and  apparently  maintain  their  health  after  reducing  the 
protein  of  their  diets  to  less  than  half  of  the  usual  amount.  No 
direct  benefit  could  be  claimed  for  this  reduction  except  that 
some  of  the  men  believed  that  they  felt  better  and  fitter  and 
more  inclined  for  work,  an  improvement  which  admits  of  no 
quantitative  measurement  because  of  the  psychological  elements 
involved.  Although  these  observations  were  conducted  with 
all  the  care  and  accuracy  of  the  highly  trained  scientist,  they 
have  been  considered  quite  inadequate  to  justify  the  claim  that 
man  takes  too  much  protein.  The  observations  have,  neverthe- 
less, been  of  immense  value  in  compelling  a  careful  review  of 
the  evidence  that  the  proportion  of  protein  which  habit  has  pre- 
scribed as  being  the  X)roper  one  for  us  to  take,  is  really  the  most 
suitable  for  our  daily  needs. 

There  are,  however,  differences  in  the  prolein  content  of  the 
diet  according  to  the  race  and  environment.  This  lias  been  as- 
certained by  compiling  the  stardard  diet  for  a  community,  that 

99 


100  HUMAN   PHYSIOLOGY. 

is,  measuring  the  exact  quantities  of  protein  and  carbohydrate 
in  the  diets  which  the  people  are  accustomed  to  live  on,  and  aver- 
aging the  results.  One  remarkable  outcome  of  such  statistical 
work  has  been  to  show  that  for  peoples  living  under  approxi- 
mately the  same  conditions  as  regards  climate  and  amount  of 
daily  muscular  work,  the  average  daily  requirement  of  calories, 
carbon  and  nitrogen  works  out  pretty  much  the  same,  although 
there  may  be  some  diversity  in  the  proportions  of  protein  and 
carbohydrate.     The  following  table  shows  this: 


Type  of  individuals.        Protein 

Fat 

Carbo. 

Total  Cal. 

C. 

N. 

gm. 

gm. 

gm. 

•gm. 

gm. 

Average  workman  in 

Germany,    20   years    age. 

118 

56 

500 

3,045 

328 

18.8 

German  soldier  in  the 

field  

151 
133 

46 
115 

522 
429 

3,190 
3,400 

340 

24 

British  soldier  in  peace... 

21.3 

Russian  soldier  in  vs^ar  (Man- 

churian  campaign)   .... 

187 

27 

775 

4,900 

30 

Professional  man   

100 

100 

240 

2,324 

230 

16 

Such  figures  can  be  compiled  with  tolerable  accuracy  because 
the  diet  is  under  control.  It  is  of  course  more  difficult  to  collect 
sufficiently  accurate  data  regarding  the  diets  of  civilians,  but  it 
is  safe  to  say  that  the  average  city  dweller  in  temperate  zones 
derives  his  daily  requirement  of  15  gm.  nitrogen  in  95  gm. 
of  protein,  which  also  yields  60  gm.  of  the  required  250  gm.  car- 
bon. This  deficit  he  might  supply  either  from  fats  or  carbohy- 
drates, the  actual  proportion  depending  on  availability  and  price. 
It  should  be  particularly  noted  that  the  proportion  of  protein  is 
very  much  increased  whenever  strenuous  muscular  work  has  to 
be  performed.  Now  the  question  is,  do  such  statistical  studies 
substantiate  Chittenden's  claim  that  the  protein  which  we  are 
accustomed  to  consume  could  profitably  be  reduced?  They  cer- 
tainly do  not.  Let  us  for  a  moment  consider  the  health  condition 
and  physical  development  of  communities  such  as  the  Bengalis 
of  Lower  Bengal,  who  live  largely  on  rice  and  take  only  a  little 
less  in  the  way  of  protein  than  the  amount  Chittenden  would 
have  us  take.  Their  body  weight,  chest  measurement  and  muscular 
development  are  distinctly  inferior  to  those  of  the  natives  of 


DIETETICS.  101 

Eastern  Bengal,  who,  nevertheless,  belong  to  the  same  race  as 
the  lower  Bengalis,  but  differ  from  them  in  taking  more  protein 
in  their  food.  Not  only  this,  but  the  loAver  Bengalis  are  in  every 
sense  of  the  word  half  starved,  and  are  very  prone  to  disease, 
especially  of  the  kidneys,  the  very  type  of  disease  to  which  we 
are  told  excessive  protein  consumption  must  predispose.  Dia- 
betes is  also  very  prevalent  amongst  these  people,  probably  be- 
cause of  the  enormous  quantities  of  sugar-yielding  food  (car- 
bohydrates) which  they  are  compelled  to  eat  in  order  to  pro- 
vide sufficient  calories  for  life.  Mentally,  they  are  a  very  in- 
ferior race.  This,  then,  is  an  experiment  on  a  much  grander 
scale  than  Chittenden's,  and  what  of  the  results?  It  is  for- 
tunate that  most  of  Chittenden's  subjects  "through  force  of 
circumstances"  have  returned  to  their  old  dietetic  habits. 

Exactly  concordant  results  have  been  obtained  when  attempts 
have  been  made  to  reduce  the  protein  in  the  dietaries  of  public 
institutions  such  as  prisons,  alms  houses,  etc.  There  has  invari- 
ably been  a  distinct  increase  in  the  sick  list,  especially  of  such 
diseases  as  pneumonia,  tuberculosis,  etc.  And  if  we  seek  for 
evidence  of  an  opposite  nature,  we  do  not  find  that  excessive 
protein  ingestion  is  fraught  with  any  evil  consequences  to  the 
community.  Thus  the  Eskimo  takes  five  times  more  protein  than 
the  Bengali  and  two  and  one-half  times  more  than  the  European, 
yet  he  is  peculiarly  free  from  "uric  acid"  diseases;  and  his 
physical  endurance  and  his  power  of  withstanding  cold  are  ex- 
traordinary. 

There  are  a  great  many  secondary  factors,  such  as  availability, 
taste,  etc.,  that  determine  the  average  diet  of  a  community,  but 
the  main  determining  factors  are  instinct  and  experience.  In 
the  struggle  for  supremacy  of  one  race  over  another,  we  may 
assume  that  adequacy  of  diet  has  been  a  determining  factor, 
and  that  the  average  which  is  takeii  usually  represents  that 
which  conduces  to  the  greatest  efficiency. 

We  have  dealt  at  some  length  on  these  questions  because  of 
their  great  practical  importance,  and  because  they  show  us  that 
in  the  matter  of  the  protein  content  of  our  diet,  as  in  that  of  all 
other  animal  functions,  there  comes  into  play  the  principle  of 


102  HUMAN   PHYSIOLOGY. 

the  ' '  factor  of  safety. ' '  "We  have  two  lungs,  although  it  is  quite 
possible  to  live  with  one  only,  two  kidneys,  although  one  will 
usually  suffice ;  and  so  with  our  food ;  we  could  get  along  for 
some  time  with  about  half  of  the  protein  which  we  take,  but  at 
the  constant  risk  of  a  deficiency,  for  should  physical  exhaustion 
occur,  a  reserve  of  building  stones  ought  to  be  available  to  re- 
store the  tissue  which  has  been  consumed.  Instead  of  the  excess 
of  protein  throwing  a  strain  on  the  organism,  the  contrary  is 
the  case,  for  it  is  indisputably  a  greater  strain  for  the  tissues  to 
have  to  construct  new  building  stones  than  to  use  those  sup- 
plied ready  made  in  the  food. 

Another  deduction  which  we  may  draw  from  these  observa- 
tions is  that  more  protein  should  be  taken  when  its  source  is 
mainly  vegetable  food  than  when  it  is  animal.  On  the  other 
hand,  there  is  nothing  to  indicate  that  one  kind  of  animal  pro- 
tein possesses  any  advantages  over  another;  flesh  protein,  milk 
protein,  egg  protein  are  practically  of  equal  dietetic  value,  and 
with  regard  to  which  varieties  of  meats — whether  light  or  dark 
— are  most  nutritious,  all  we  can  say  is  that  any  differences  that 
may  be  thought  to  exist  are  not  due  to  differences  in  the  chemical 
nature  of  the  proteins  which  they  contain,  but  depend  on  their 
flavor  and  digestibility.  There  are  more  fads  and  fancies  about 
which  meats  are  nutritious  and  which  are  not  so  than  would  fill 
a  volume,  but  after  all  the  whole  question  is  one  of  flavor.  Man 
digests  best  what  he  likes  best,  and  he  thrives  best  when  digestion 
is  good.  A  sound  knowledge  of  the  principles  of  dietetics  is 
of  no  less  importance  for  the  dentist  than  the  physician;  but 
there  are  no  simple  rules  by  which  the  most  suitable  diet  for 
each  individual  can  be  prescribed.  Many  factors  besides  the 
nutritive  values  of  the  food  must  be  considered,  and  the  old 
adage  should  never  be  forgotten,  that  "one  man's  food  is  an- 
other man's  poison." 

Very  practical  conclusions  may  be  drawn  from  these  observa- 
tions regarding  the  most  suitable  diet  for  the  city  dweller.  It  is 
evident  that  we  are  now-a-days  in  possession  of  a  sufficient 
amount  of  scientific  information  regarding  both  the  daily  require- 
ments of  the  body  and  the  ability  of  the  various  foodstuffs  to 


DIETETICS.  103 

fulfill  these  requirements,  to  compute,  from  the  market  prices  of 
foods,  how  much  it  should  take  per  diem  for  an  individual,  or  a 
family  of  individuals,  to  live  healthfully  and  economically.  The 
day  will  surely  come  when,  through  the  medium  of  schools  and 
the  press,  everyone  will  know  what  we  may  call  the  fundamentals 
of  dietetics,  namely:  (1)  that  a  man  of  sedentary  occupation 
(the  ordinary  city  clerk)  requires  daily  2,600  calories,  and  a 
laboring  man,  at  least  3,000  calories.  (2)  That  at  least  .5  per 
cent  of  the  calories  should  be  provided  in  protein  food  of  animal 
origin  (meats,  milk)  with  10  per  cent  or  more  as  other  protein 
(bread,  oatmeal,  etc.). 

To  enable  the  housewife  to  purvey  the  necessary  food  to  meet 
these  requirements,  she  must  therefore  become  familiar  with  the 
calorie  value  and  the  percentage  of  protein  in  the  different 
classes  of  protein  foods,  and  of  the  caloric  values  of  other  great 
staples  of  diet.  Canned  foods  will  no  doubt  some  day  have 
printed  on  the  label :    ' '  This  can  contains  ....  calories,  of  which 

....  per  cent  are  in  proteins  of  grade "    And  this  is  no 

Utopian  idea ;  it  is  practical  common  sense.  The  adoption  of 
such  a  scheme  is  far  more  likely  to  be  the  solution  of  the  problem 
of  the  high  cost  of  living  than  anything  else,  for,  indeed,  it  is  not 
so  much  the  high  cost  of  living  as  it  is  the  cost  of  high  living 
that  troubles  us.  We  demand  business  efficiency  in  our  manufac- 
turing organizations,  and  yet  we  are  inclined  to  ridicule  as  im- 
practical any  attempts  at  nutritive  efificieney  in  the  animal  organ- 
ization which  is  our  own  body.  Not  only  the  principles  of 
dietetics,  but  the  details  as  well  are  now  so  thoroughly  under- 
stood that  their  application  in  the  feeding  of  the  masses  is  only 
a  matter  of  education.  Dietery  impostures  of  the  meanest  de- 
scription, often  hiding  behind  a  "bluff"  of  scientific  knowledge, 
are  of  course  the  most  serious  enemies  we  shall  have  to  face  in 
spreading  the  knowledge.  It  will  be  the  duty  of  physicians,  of 
dentists,  and  of  the  educated  classes  to  offset  this  commercial 
brigandage  by  spreading  the  gospel  of  food  efficiency. 

As  illustrating  the  food  efficiency,  in  relationship  to  cost  we 
may  take  the  following  table  from  the  menu  of  a  well-known 
restaurant  company: 


104 


HUMAN   PHYSIOLOGY. 


Cost 

Calories 

Calories 

Cost 

Bread  

in  cents 
per  portion 

5 

Total  *-       %  in 
protein 

933                 12 

343                   5 

868                 12 

for  5  cents 

933 

337 

276 

in  cents 

per  1000 

calories 

5 

Apple  pie 

Boston  pork 
and  beans   

..       5 
.      15 

15 
18 

Ham  sandwich  , 

...       5 

212                 20 

198 

30 

Corn  beef  hash. 

..     15 

538                 14 

170 

30 

Beef  stew 

..     15 

641                 25 

199 

32 

Club  sandwich.  . 

...      25 

438                 20 

82 

61 

Sliced  pineapple 
Mayonaise    

..        5 

..      20 

36                   8 
53                 16 

36 
13 

138 
35 

(Lusk) 

The  above  table  is  not  by  any  means  from  a  cheap  restaurant. 
By  economy  and  judicious  purchasing  it  is  possible  even  in  New 
York  to  purchase,  for  8  cents,  1,000  calories  having  the  proper 
proportion  of  protein,  so  that  a  working  man  may  easily  cover 
his  dietetic  requirements  for  25  cents  a  day,  exclusive  of  the 
cost  of  cooking.  All  he  spends  above  this  is  for  personal  taste 
and  relish. 

Chemistry  of  the  Commoner  Foodstuffs. 

The  accompanying  diagram  (Fig.  9)  indicates  the  composition 
of  some  of  the  commoner  foods  and  is  self-explanatory.  There 
are. certain  foodstuffs  concerning  which  a  little  more  detail  may 
however  be  advisable. 

Wheat  Flour,  besides  a  large  amount  of  starch,  contains  two 
proteins,  glutein  and  gliadin.  When  the  flour  is  mixed  with 
water  and  then  kneaded,  it  forms  dough,  because  the  proteins 
change  into  a  sticky  substance  called  gluten.  As  dough  the  flour 
is  not  a  suitable  food,  because  the  digestive  juices  cannot  pene- 
trate it.  To  render  it  digestible  the  dough  must  be  made  porous 
and  this  is  accomplished  by  causing  bubbles  of  carbon  dioxide 
gas  to  develop  in  it,  either  by  mixing  it  with  baking  powder 
which  is  composed  of  a  bicarbonate  and  an  organic  acid  (tar- 
taric) or  by  keeping  it  in  a  warm  place  with  yeast,  which  fer- 
ments the  sugar  that  is  present.  The  sugar  is  developed  from 
the  starch  by  the  action  of  the  diastase  (see  p.  44)  present  in 
the  flour. 


10  20  30  40  50  60  70  80  00         100 


1  Whole   milk. 
Skim   milk. 
( 'ream. 

Cheese. 

Butter. 

Egg. 

Average  meat  (raw). 

Average   mutton    (raw). 
Average   pork    (raw). 

Fi.sh — llounrter    (raw). 

Bacon. 

Wheat   l)read 

Oats. 

liice. 


.\sh   and    water. 


Protein  of  2n(l   Qunlity. 


Em 


r'rotein  of  1st  Qunlity. 


Carbohydrate. 


Fat. 


Calories. 


Fig.  '.).- — l.>ietetic  chart,  showing  ttie  percentage  amounts  of  llie  vai'ious 
liroximate  pr-incii)les  Hndicated  iiy  the  shiided  ;irc:is)  :in<l  the  calories  (indi- 
ciitr-d  in  red)  yielded  by  Iturning  1  11).  of  the  runnriomi  fuod.si  uffs.  The  num- 
ber.'; to  the  left  represent  the  cjiloric  values  and  the  nairies  to  tlw  right,  the 
food    in    i|Uestion. 


DIETETICS.  105 

"When  the  yeast  has  been  allowed  to  act  for  some  time,  or  if 
baking  powder  was  used,  when  the  gas  formation  has  ceased,  suit- 
able portions  (loaves)  of  dough  are  placed  in  the  oven.  The  heat 
causes  the  inclosed  bubbles  of  gas  to  expand  so  that  the  whole 
m^ss  becomes  aerated  and  further  increase  of  temperature  acts 
on  the  proteins  and  starches  on  the  surface  coagulating  the  for- 
mer and  converting  the  latter  into  dextrins.  The  crust  is  thus 
formed.  Brown  bread  is  made  from  wheat  from  which  all  the 
husk  has  not  been  removed.  There  are  two  possible  advantages 
of  this  over  white  bread,  namely,  the  husks  act  as  a  mild  laxative 
and  they  seem  to  contain  traces  of  vitamines  (see  p.  121). 

Other  Cereals. — These  include  maize  or  Indian  corn,  oatmeal 
and  rice,  and  differ  from  wheat  in  that  their  proteins  do  not  form 
gluten  when  mixed  with  water.  They  cannot  therefore  be  formed 
into  bread  unless  they  be  mixed  with  some  wheat  flour.  They  are 
relatively  rich  in  ash,  and  maize  contains  a  large  proportion  of 
fat.  When  rice  composes  a  large  proportion  of  the  diet,  as  is  the 
case  in  tropical  countries,  the  unpolished  variety  should  be  used 
to  supply  the  vitamines.  When  the  diet  is  a  mixed  one,  however, 
danger  of  an  insufficiency  of  vitamines  cannot  exist.  As  has 
been  already  explained,  the  protein  of  cereals  is  not  of  first  qual- 
ity, because  it  does  not  contain  all  of  the  amino  acids  (building 
stones)  of  tissue  proteins. 

Milk  and  Milk  Preparations. — ^Whole  milk  is  as  nearly  as 
possible  a  perfect  food,  for  its  protein  is  of  the  first  quality  and 
it  contains  a  sufficiency  of  fats  and  carbohydrates  for  the  growth 
of  the  tissues.  Where  muscular  exercise  must  also  be  performed, 
carbohydrates  should  be  added  to  the  milk,  and  this  is  best  ac- 
complished by  the  use  of  cereals.  Milk  is  an  economical  food, 
for  one  quart  nearly  equals  in  nutritive  value  a  pound  of  steak 
or  eight  or  nine  eggs,  and  is  easily  digested  and  assimulated,  but 
somewhat  constipating.  The  chief  protein  of  milk  is  caseinogen 
(phospho  protein)  and  is  characterized  by  being  precipitated 
by  weak  acids  and  by  tlie  action  of  gastric  juice.  When  milk  sours 
some  of  the  milk  sugar,  or  lactose,  becomes  converted  by  bacterial 
action  into  lactic  acid  and  this  precipitates  caseinogen.  When 
an  extract  of  the  mucous  membrane  of  the  stomach  is  added  to 


106  HUMAN   PHYSIOLOGY. 

milk  and  the  mixture  kept  warm,  the  clot  which  forms  is  called 
casein.  By  separating  the  casein  and  allowing  it  to  stand  for 
some  time  ferments,  derived  from  moulds  and  bacteria,  act  on 
it  to  produce  cheese.  The  cheese,  besides  casein,  contains  much 
fat  and  mineral  matter.  Cheddar  cheese  is  especially  rich  in  fat. 
Cheese  is  a  very  concentrated  article  of  diet  and  when  taken  in 
moderation  is  thoroughly  digested  and  assimilated. 

Cream  consists  of  the  milk  fats  with  some  of  the  constituents 
of  milk.  It  is  the  most  easily  assimilated  of  all  the  fats  and  is 
hence  very  nutritious.  When  sweetened,  flavored  and  frozen  it 
forms  ice  cream,  which  should  not  be  regarded,  as  it  usually  is, 
as  a  luxury,  but  as  a  highly  nutritious  food.  It  should  not  there- 
fore surprise  the  indulgent  parent  when  a  child  refuses  food 
after  visiting  the  corner  pharmacy.  On  standing,  cream  ripens 
(undergoes  change  due  to  bacterial  growth),  and  the  fat  can 
be  made  to  separate  as  butter.  There  is  no  foodstuff  that  con- 
tains more  calories  than  butter,  and  it  also  contains  certain  vi- 
tamines.  The  fluid  from  which  the  butter  separates,  hutter- 
milk,  contains  practically  no  fat  and  is  acid  to  the  taste  because 
of  bacterial  action  on  the  lactose  producing  lactic  acid.  Its  in- 
fluence on  the  nature  of  bacterial  growth  in  the  intestines  has 
already  been  referred  to. 

Eggs. — The  only  point  we  need  emphasize  is  the  much 
greater  percentage  of  fat  substances  (lipoids)  in  the  yolk  than 
in  the  white.  One  dozen  eggs  equals  in  food  value  two  pounds 
of  meat.    Eggs  are  therefore  more  costly  than  milk. 

Meats. — The  building  stones  of  the  protein  molecule  of  meat, 
for  reasons  which  are  obvious,  are  more  nearly  identical  with 
those  of  the  tissues  of  man  than  are  those  of  any  other  food.  The 
carbohydrate  is  however  insufficient  in  amount,  for  which  rea- 
son we  take  potatoes  with  meat.  The  flavors  of  different  meats 
depend  largely  on  the  extractive  substances  which  they  contain. 
These  include  creatin  and  purin  substances.  When  a  decoction 
of  meat  is  evaporated  to  small  bulk,  after  precipitating  all  of  the 
protein,  meat  extract  is  prepared,  which,  like  coffee  or  tea,  has 
no  nutritive  value  but  acts  as  a  mild  stimulant  ( caffein  and  thein 
are  chemically  very  closely  related  to  the  purin  bodies  of  meat 


DIETETICS.  107 

extract).  Clear  soups  are  mainly  dilute  solutions  of  meat  ex- 
tractives, but  in  beef  tea,  if  properly  made,  there  is  much 
meat  protein. 

Other  Foods  and  Condiments. — Although  green  vegetables 
and  salads  consist  very  "largely  of  water,  they  are  very  important 
articles  of  diet,  because  they  contain  cellulose,  which  serves  to 
increase  the  bulk  of  the  intestinal  contents — to  serve  as  ballast, 
as  it  were — and  prevent  constipation  by  keeping  the  intestinal 
musculature  active.  Some  vegetables,  such  as  spinach,  are 
especially  important  since  they  contain  iron.  Salads  have  a 
further  importance  because  of  the  oil  taken  with  them.  The  rel- 
ishes and  the  condiment  flavors  are  by  no  means  insignificant 
adjuncts  of  diet,  for  they  give  the  relish  to  food  without  which 
digestion  is  likely  to  be  inefficient.  This  most  important  prop- 
erty of  diet  has  been  sufficiently  insisted  upon  elsewhere. 


CHAPTER  XI. 
SPECIAL  METABOLISM. 

But  we  must  now  return  to  the  more  theoretical  aspects  of  our 
subject.  We  will  proceed  to  trace  out  very  briefly  the  interme- 
diary stages  in  metabolism  through  which  proteins,  fats  and  car- 
bohydrates have  to  pass  in  order  to  yield  the  energy  required 
to  drive  the  animal  machine  and  to  supply  material  with  which 
to  repair  the  broken-down  tissues. 

Metabolism  of  Proteins. — ^We  must  follow  the  amino  acids 
after  their  absorption  into  the  blood  until  they  ultimately  reap- 
pear, the  nitrogen  among  the  nitrogenous  constituents  of  urine 
and  the  carbon  as  part  of  the  carbon  dioxide  of  expired  air.  In 
order  to  do  this  it  is  necessary  for  us  to  become  familiar  with 
tJie  nature  and  source  of  the  urinary  substances  which  contain 
nitrogen,  and  to  consider  some  of  the  most  important  chemical 
relationships  of  these  substances,  so  that  we  may  understand  how 
they  become  formed  in  the  body.  The  substances  in  question  are  : 
urea,  ammonia,  creatinin,  the  purin  bodies,  and  undetermined 
nitrogenous  substances.  Urea  and  ammonia  may  be  considered 
together. 

Urea  and  Ammonia. — There  is  no  doubt  that  it  is  as  ammonia 

that  the  nitrogen  of  the  amino  acids  is  set  free  in  the  organism. 

The  free  ammonia  would,  however,  be  highly  poisonous,  so  that 

it  immediately  becomes  combined  with  acid  substances  to  form 

harmless  neutral  salts.     The  acid  which  is  ordinarily  used  for 

this  purpose  is  carbonic,  of  which  there  is  always  plenty  in  the 

blood  and  tissue  juices.     The  ammonium  carbonate  thus  formed 

becomes  changed  into  urea  by  removal  of  the  elements  of  water 

from  the  molecule,  thus: 

OH       ONH4  NH.  NH, 

/  /  /    '  / 

2NH3  +  CO        =  CO       —  H2O  =  CO       —  H2O  =  CO 


\      \ 

\ 

\ 

OH       ONH, 

ONH, 

NH2 

Ammonia  Carbonic  Ammonium 

Ammonium 

Urea 

acid.          carbonate 

carbamate 

108 


THE  METABOLISM  OF  PROTEINS.  109 

The  conversion  of  ammonium  carbonate  occurs  largelj'^  in  the 
liver.  Our  evidence  for  this  is:  (1)  If  solutions  containing 
ammonium  carbonate  be  made  to  circulate  through  an  excised 
liver,  urea  is  formed.  (2)  If  this  organ  be  seriously  damaged, 
either  experimentally  or  by  disease,  less  urea  and  more  ammonia 
appear  in  the  urine.  "W.e  see  therefore  that  urea  is  formed  in 
order  to  prevent  the  poisonous  action  of  ammonia.  But  the  am- 
monia may  be  more  usefully  employed;  instead  of  being  com- 
bined with  carbonic  acid  in  order  that  it  may  be  got  rid  of,  it 
may  be  employed  to  neutralize,  and  thus  render  harmless,  any 
other  acids  that  make  their  appearance.  Thus,  it  may  be  em- 
ployed to  neutralize  the  acids  which  sometimes  result  during  the 
metabolism  of  fat,  as  in  the  disease  diabetes;  or  the  lactic  acid 
that  appears  in  the  muscles  during  strenuous  muscular  exercise ; 
or  the  acids  produced  on  account  of  inadequate  oxygenation. 
Taking  acids  by  the  mouth  has  a  similar  effect;  thus  the  am- 
monia excretion  rises  after  drinking  solutions  containing  weak 
mineral  acids. 

Ammonia  is,  of  course,  not  the  only  alkali  which  is  available 
in  the  organism  for  the  purpose  of  neutralizing  acids.  The 
fixed  alkalies,  sodium  and  potassium,  are  also  used.  Thus-,  when 
we  greatly  increase  the  proportion  of  these,  as  by  taking  alkaline 
drinks^  or  by  eating  vegetable  foods,  the  ammonia  excretion 
diminishes. 

Urea  is  an  inert  substance,  capable  of  uniting  with  acids  to 
form  unstable  salts  (urea  nitrate  and  oxalate),  and  like  other 
amino  acids,  being  decomposed  by  nitrous  acid  so  as  to  yield 
free  nitrogen.  This  latter  reaction  is  used  for  the  quantitative 
estimation  of  urea,  the  evolved  nitrogen  being  proportional  to 
the  amount  of  urea,  thus : 

NH, 

/ 

CO      +  2  HNO2  =  2  CO2  +  2  No  +  2  H2O 

\ 
NH2 

Certain  bacteria  are  capable  of  causing  urea  to  take  up  2  mole- 
cules of  water  so  as  to  form  ammonium  carbonate,  a  process 


110  HUMAN  PHYSIOLOGY. 

really  the  reverse  of  that  which  occurs  in  the  organism  and  rep- 
resented by  the  above  formulae.  This  change  occurs  in  urine  and 
accounts  for  the  ammoniacal  odor  which  develops  when  this  fluid 
is  allowed  to  stand. 

Creatinin. — This  is  very  closely  related  to  creatin,  which  is  the 
most  abundant  extractive  substance  in  muscle,  and  which  yields 
urea  when  it  is  boiled  with  weak  alkali.  These  chemical  facts 
would  lead  us  to  expect  that  some  relationship  must  exist  be- 
tween the  creatin  of  muscle  and  the  creatinin  and  urea  of  urine, 
but,  so  far,  it  has  been  impossible  to  show  what  this  relationship 
is.  One  very  important  fact  has,  however,  been  brought  to  light, 
namely,  that  creatin  makes  its  appearance  in  the  urine  when 
carbohydrate  substances  are  not  being  oxidized  in  the  body,  as 
in  starvation,  and  in  the  disease  diabetes.  This  is  one  reason  for 
the  growing  belief  that  carbohydrates  are  something  more  than 
mere  energy  materials  (see  p.  113).  The  excretion  of  creatinin 
is  so  remarkably  independent  of  the  amount  of  protein  in  the 
food  that  it  is  believed  to  represent  more  especially  the  end  prod- 
uct of  the  protein  break-down  of  the  tissues  themselves,  in  con- 
trast to  urea,  which  jDartly  represents  the  cast-off  nitrogen  of  the 
protein  of  the  food. 

PuRiN  Bodies. — These  are  of  particular  interest  because  they 
include  uric  acid,  about  which  more  nonsense  has  been  written 
than  about  any  other  product  of  animal  metabolism.  The  so- 
called  uric  acid  diathesis  is  very  largely  a  medical  myth — a  cloak 
for  ignorance.  Uric  acid  is  the  end  oxidation  product  of  the 
purin  bodies,  which  include  the  hypoxanthin  and  xanthin  of 
muscle  and  their  amino  derivatives,  the  adenin  and  guanin  of 
nuclein. 

These  relationships  are  seen  in  the  following  f ormulse : 

Oxy  purins  of  muscle |  Hypoxanthin C,H,N,0 

I  Xanthin  C^H.N^O, 

Amino  purins  of  nuclein.  .  ]  ^^^^i^ C,H,N,NH 

(Guanin    C^H.N.ONH 

Uric  acid  CsH.N.Oo 

There  are  therefore  two  sources  for  uric  acid  in  the  animal 


THE  METABOLISM  OP  PROTEINS. 


Ill 


body,  namel}',  the  muscles  and  the  nuclei  of  the  cells.  This  ex- 
plains why  the  uric  acid  excretion  increases  after  strenuous  mus- 
cular work,  and  why  it  is  much  above  the  normal  when  cellular 
break-down  is  very  excessive,  as  in  the  disease  called  leueocythe- 
mia,  in  which  there  is  an  excess  of  leucocytes  in  the  blood  (see 
p.  145).  Another  source  of  uric  acid  is  the  food  when  it  con- 
tains either  muscle  (flesh)  or  glands  (sweetbreads),  for  a  large 
proportion  (about  half)  of  the  ingested  purins  do  not  become 
destroyed  in  their  passage  through  the  organism,  but  become 
oxidized  to  uric  acid,  which  is  excreted  in  the  urine.  This  is 
called  the  exogenous  in  contrast  to  purin  produced  in  the  tissues, 
which  is  called  endogenous. 

There  is  only  a  trace  of  uric  acid  in  the  urine  of  mammals,  but 
in  birds  and  reptiles  most  of  the  nitrogen  is  present  in  this  form. 
The  reason  is  that  in  these  animals  it  is  important  to  have  semi- 
solid, instead  of  fluid  excreta,  so  that  the  urea  which  results  from 
protein  metabolism  becomes  converted  into  uric  acid,  which, 
either  free  or  as  salts,  is  relatively  insoluble.  Uric  acid  is  chemi- 
cally a  diureide,  that  is  to  say,  it  consists  of  two  urea  molecules 
linked  together  by  a  chain  of  carbon  atoms.  The  chain  of  carbon 
atoms  is  furnished  by  substances  not  unlike  lactic  acid  and  the 
synthesis  occurs  in  the  liver.  If  this  organ  be  removed  from  the 
circulation  in  birds,  such  as  geese,  in  which  the  operation  is 
comparatively  easy,  a  very  large  part  of  the  uric  acid  in  the  urine 
becomes  replaced  by  ammonium  lactate. 

The  relative  insolubility  of  uric  acid  and  its  salts,  which  we 
have  already  referred  to,  makes  it  apt  to  become  precipitated  in 
urine,  especially  on  standing.  It  forms  the  orange  reddish  de- 
posit, so  frequently  observed  in  summer,  when  on  account  of  per- 
spiration the  urine  does  not  contain  as  much  water  as  usual. 
Such  deposits  do  not  therefore  indicate  that  there  is  an  excess  of 
uric  acid  in  the  blood,  but  merely  that  enough  water  is  not  being 
excreted  to  dissolve  the  usual  amount  of  urates.  Sometimes  the 
urate  becomes  deposited  in  the  joint  cartilages,  particularly  in 
those  of  the  great  toe,  causing  local  swelling  and  redness  and 
great  pain.  This  is  gout,  and  it  may  be  most  effectually  treated 
by  drinking  large  quantities  of  alkaline  fluids,  and  eliminating 


112  HUMAN  PHYSIOLOGY.      • 

from  the  dietary  such  foodstuffs  as  meats  and  sweetbreads,  which 
yield  exogenous  purins.  As  we  have  said,  there  is  no  reason  to 
believe  that  any  other  diseases  besides  gout  are  due  to  an  ex- 
cess of  uric  acid  in  the  blood. 

Besides  the  above  there  are  traces  of  other  nitrogenous  sub- 
stances in  the  urine,  such  as : 

1.  Hippurie  acid,  which,  as  its  name  signifies,  is  very  abun- 
dant in  the  urine  of  the  horse  and  other  herbivora,  and  which  is 
the  excretory  product  of  the  aromatic  substances  which  the  food 
of  these  animals  contains. 

2.  Cystin,  an  amino  acid  containing  sulphur. 

3.  Pigments  and  mucin. 

The  exact  significance  of  the  end  products  of  nitrogenous  met- 
aholism  has  been  very  beautifully  demonstrated  by  Folin,  of 
Harvard.  The  observations  were  made  on  several  men  who  lived 
for  some  days  on  a  diet  rich  in  protein  (but  containing  no  purin- 
containing  foodstuffs),  and  then  on  one  which  was  very  poor  in 
protein.  The  problem  was  to  see  how  each  of  the  nitrogenous 
constituents  behaved  during  the  two  periods,  both  absolutely 
and  in  relation  to  the  total  amount  of  nitrogen  excreted.  In  or- 
der to  show  the  latter  relationship  the  results  are  given,  as  in  the 
following  table,  not  as  urea,  etc.,  but  as  urea-nitrogen,  etc. : 

On  the  protein-rich  On  the  protein- 
diet  poor  diet 

Quantity  of  urine 1170  c.  c.  385  c.  c. 

Total    nitrogen    16.8     gm.  3.6     gm. 

Urea-nitrogen    14.7     gm.   (87.5)  2.2     gm.   (61.7) 

Ammonia-nitrogen     0.49  gm.   (3.0)  0.42  gm.   (11.3) 

Uric  acid-nitrogen    0.18  gm.   (1.1)  0.09  gm.   (2.5) 

Creatinin-nitrogeji    0.58  gm.   (3.6)  0.60  gm.   (17.2) 

Undetermined  nitrogen.        0.85  gm.   (4.9)  0.27  gm.  (7.3) 

The  figures  in  parentheses  represent  the  percentage  which  the 
nitrogen  of  each  substance  furnishes  of  the  total  amount  of  nitro- 
gen excreted.  It  will  be  seen  that  urea  decreases  on  the  poor 
diet  relatively  more  than  total  nitrogen,  thus  indicating  that  it 
comes  partly  from  proteins  in  the  food  (exogenous)  and  partly 


THE  METABOLISM  OF  PROTEINS.  113 

from  the  organism  itself  (endogenous).  This  result  leads  us  to 
infer  that  most  of  the  amino  substances  of  protein  foods  which 
are  not  required  as  building  stones  for  the  tissues  are  broken 
down  so  as  to  yield  ammonia,  which  is  excreted  as  exogenous  urea 
in  the  urine,  but  that  the  amino  acids  that  are  really  appropri- 
ated by  the  tissues,  although  they  may  also  produce  some  urea 
(endogenous),  cause  other  end-products  to  be  formed.  The  most 
important  of  these  endogenous  bodies  is  evidently  creatinin,  for, 
as  will  be  seen  from  the  above  table,  this  substance  is  excreted  in 
the  same  absolute  amount  during  both  the  starvation  and  the 
protein-rich  periods. 

Direct  evidence  that  this  conclusion  is  correct  has  been  ob- 
tained by  examination  of  the  blood  and  muscles  for  amino  bodies, 
ammonia  and  urea.  The  results  have  shown  that  the  amino 
acids  absorbed  from  the  intestine  are  carried  through  the  liver 
into  the  systemic  blood,  which  transports  them  to  the  muscles, 
where  those  that  are  not  required  for  building  up  the  tissues 
are  broken  down  into  ammonia  and  a  carbonaceous  residue,  which 
is  then  burned  just  exactly  as  if  it  were  carbohydrate  or  fat. 
The  useless  ammonia  becomes  converted  into  urea  in  the  manner 
already  described,  either  in  the  muscles  themselves,  or  by  being 
carried  to  the  liver,  which,  as  we  have  seen,  possesses  to  a  very 
high  degree  the  power  of  producing  urea. 

The  Relative  Importance  of  Proteins,  Fats  and  Carbohy- 
drates in  Metabolism. — The  metabolism  of  fats  and  carbohy- 
drates, with  regard  both  to  their  importance  as  builders  of  living 
tissues  and  the  type  of  their  metabolism,  is  very  different  from 
that  of  proteins.  That  carbohydrates  and  fats  are  less  impor- 
tant in  the  animal  economy  than  proteins  is  evidenced  by  the 
fact  that  we  can  live  perfectly  well  on  protein  food  alone,  but 
jiot  on  either  of  the  others.  This  does  not,  however,  justify  us 
in  concluding  that  carbohydrates  and  fats  are  merely  materials 
which  are  oxidized  by  the  tissues  for  the  purpose  of  producing 
energy,  fuel  as  it  were,  and  which  can  be  dispensed  with.  They 
are  more  than  this,  for  no  cell,  in  however  starved  a  condition 
it  may  be,  is  entirely  free  from  either  of  them,  thus  indicating 
that  they  must  have  been  produced  out  of  protein  itself.    Pro- 


114  HUMAN    PHYSIOLOGY. 

teins  are  no  doubt  the  most  important  ingredients  of  cells,  but 
fats  and  carbohydrates  are  indispensable  also. 

As  reserve  materials,  striking  differences  exist  among  the 
three  foodstuffs.  Proteins  are  of  little  value  in  this  regard  f ol*,  as 
we  have  seen,  very  little,  if  any,  can  become  laid  down  in  the 
tissues  when  excess  is  taken  as  food ;  on  the  contrary,  all  that  is 
not  required  is  thrown  out  of  the  body,  and  when  the  food  sup- 
ply is  cut  off,  as  in  starvati'&n,  the  protein  is  spared  as  much  as 
possible  (see  p.  92).  Carbohydrates  are  very  readily  depos- 
ited as  a  starch-like  substance,  called  glycogen,  and  this  reserve 
is  the  first  to  be  called  on,  not  only  in  starvation,  but  also  when 
muscular  work  is  performed.  It  may  be  considered  as  the  most 
immediately  available  material  for  combustion  in  the  organism, 
but  the  limits  of  its  storage  are  restricted  in  man  to  some  hun- 
dreds of  grams,  which,  as  we  have  seen,  soon  become  used 
up  in  starvation.  Fat  is  pre-eminently  the  storage  material,  and 
the  supply  may  serve  in  man  to  furnish,  along  with  a  little  pro- 
tein, enough  fuel  for  several  weeks'  existence. 

The  relative  importance  of  the  three  foodstuffs  is  shown  in  the 
extent  to  which  each  is  used  in  the  metaholism  during  muscular 
exercise.  When  there  is  an  abundant  store  of  glycogen,  the 
energy  is  entirely  derived  from  this  source ;  when  there  is  little 
glycogen  but  much  fat,  it  is  fat  that  is  burned,  and  when  neither 
of  these  is  abundant  but  much  protein  is  being  taken  with  the 
food,  or  the  animal  is  reduced  to  living  on  its  own  tissues,  as  in 
starvation,  it  is  protein.  In  other  words,  the  type  of  metabolism 
occurring  during  muscular  work  is  the  same  as  that  which  imme- 
diately preceded  it ;  the  only  change  is  in  the  extent  of  the  com- 
bustion, not  in  the  nature  of  the  fuel  employed. 


CHAPTER  XII. 
SPECIAL  METABOLISM  (Cont'd). 

Metabolism  of  Fats. — Fats  are  absorbed  hy  the  lacteals  and 
discharged  into  the  blood  of  the  left  subclavian  vein  through 
the  thoracic  duct.  They  are  carried  to  various  parts  of  the  body 
and  gain  entry  into  the  cells,  in  the  protoplasm  of  which  they 
become  deposited.  This  process  occurs  extensively  in  the  sub- 
cutaneous connective  tissues,  between  the  muscles,  and  retroperi- 
toneally  around  the  kidney  (the  suet).  The  fat  which  is  thus 
deposited  possesses  more  or  less  the  same  qualities  as  the  fat  of 
the  food.  Thus,  when  the  only  fat  taken  over  a  long  period  of 
time  is  one  with  a  very  low  melting-point,  such  as  oil,  the  fat 
deposited  in  the  tissues  is  likely  to  be  oily  in  character,  whereas 
it  is  stiff  after  feeding  with  a  high  melting-point  fat,  such  as 
mutton  fat.  This  similarity  between  the  tissue  fat  and  that'  of 
the  food  becomes  very  striking  when  the  animal  has  been  sub- 
jected to  a  preliminary  period  of  starvation  and  then  fed  for 
some  weeks  with  a  large  excess  of  the  particular  fat  and  as  little 
carbohydrate  and  protein  as  possible.  Fat  in  the  food  is  of 
course  not  the  only  source  of  the  fat  in  the  tissues.  It  is  also 
formed  out  of  carbohydrates,  a  fact  which  is  well  known  to 
farmers,  who  fatten  their  stock  by  feeding  them  with  maize 
and  other  starchy  grains,  and  to  physicians,  who  reduce  their 
corpulent  patients  by  restricting  carbohydrate  foods.  The  fat 
thus  deposited  has  the  chemical  characteristics  of  the  fat  which 
is  peculiar  to  that  animal.  It  is  almost  certain  that  there  is  ordi- 
narily no  formation  of  fat  out  of  protein  in  the  higher  animals. 

The  fat  thus  deposited  in  the  tissues  may  remain  for  a  long 
time,  but  ultimately  it  is  again  taken  up  by  the  blood  and  car- 
ried to  whatever  active  tissue  requires  it  as  fuel.  Before  being 
thus  burnt,  it  splits  into  glycerine  and  fat  acid  (see  p.  75).  The 
fat  acid  possibly  undergoes  some  preliminary  change  in   the 

115 


116  HUMAN  PHYSIOLOGY. 

liver;  in  any  case,  the  long  chain  of  carbon  atoms  of  which  we 
have  seen  the  fat  molecule  to  be  composed  (see  p.  24)  becomes 
oxidized  (burnt),  not  all  at  once  but  piece  by  piece,  two  carbon 
atoms  being  split  off  at  a  time.  If  the  fat  acid  chain  originally 
contained  an  even  number  of  carbon  atoms,  the  oxidation 
process  may  stop  short  when  there  are  yet  four  carbon 
atoms  in  the  chain,  thus  producing  oxybutyric  acid 
(CH3CHOHCH2COOH).  This  imperfect  metabolism  of  fat  oc- 
curs in  severe  cases  of  diabetes  and  often  causes  death.  It  also 
occurs  in  carbohydrate  starvation,  and  indicates,  more  clearly 
than  any  thing  else,  that  even  carbohydrates  are  essential  for  life. 

Metabolism  of  Carbohydrates. — ^It  will  be  remembered  that 
these  include  the  starches  and  the  sugars,  and  that  during  diges- 
tion they  are  all  hydrolyzed  to  dextrose  or  Isevulose,  as  which 
they  are  absorbed  into  the  blood  of  the  portal  vein.  This  ab- 
sorption is  rapid,  so  that  a  striking  increase  in  the  percentage 
of  sugar  occurs  in  the  blood  of  the  portal  vein  shortly  after  the 
food  has  been  taken.  Most  of  this  excess  of  sugar  does  not  imme- 
diately gain  entry  to  the  blood  of  the  systemic  circulation,  how- 
ever, because  it  is  retained  by  the  liver.  For  this  purpose  the 
liver  cells  convert  the  sugar  into  the  starch-like  substance,  glyco- 
gen, which  becomes  deposited  in  their  protoplasm  as  irregular 
colloidal  masses,  which  stain  with  iodine  and  carmine.  The  liver 
does  not  manage  in  this  way  to  remove  all  of  the  excess  of  sugar 
from  the  portal  blood,  so  that,  even  in  a  healthy  animal,  there 
is  a  distinct  postprandial  increase  of  sugar,  or  hyperglycaemia,  as 
it  is  called,  in  the  systemic  blood.  If  too  much  sugar  passes  the 
liver  it  causes  so  marked  a  postprandial  hyperglycsemia  that 
some  sugar  escapes  into  the  urine,  thus  causing  glycosuria.  This 
is  one  of  the  early  symptoms  of  diabetes,  and  its  occurrence 
furnishes  us  with  a  warning  that  less  carbohydrates  should  be 
given  in  the  food.  If  the  warning  be  heeded,  the  severer  form 
of  the  disease  will"  very  probably  be  staved  off. 

The  glycogen  deposited  in  the  liver  stays  there  until  the  per- 
centage of  sugar  in  the  systemic  blood  begins  to  fall  below  the 
normal  level  (which  in  man  is  about  0.1  per  cent),  when  it 
becomes  reconverted  into  sugar,  which  is  added  to  the  blood. 


THE  METABOLISM  OF   CARBOHYDRATES.  117 

Tlie  reason  why  the  sugar  in  the  systemic  blood  tends  to  fall  is 
that  the  tissues,  especially  the  muscles,  are  using  it  up  as  fuel. 
If  so  much  sugar  is  taken  that  the  storage  capacity  of  the  liver 
is  overstepped,  the  excess  of  sugar  is  carried  by  the  systemic 
blood  to  the  tissues,  where  much  of  it  may  be  changed  into  fat. 
The  glycogenic  function  of  the  livet,  as  the  above  process  is 
called,  is  analogous  to  the  starch-forming  function  of  many 
plants,  such  as  potatoes.  Of  the  sugar  which  is  formed  in  the 
green  leaves  of  these  plants,  some  is  immediately  used  for  build- 
ing up  other  substances,  the  remainder  being  converted  into 
starch,  which  becomes  deposited  in  the  roots,  etc.,  until  it  is 
required  (as  during  the  second  year's  growth),  when  it  is  grad- 
ually reconverted  into  sugar. 

Besides  carbohydrates  it  is  known  that  proteins  form  glyco- 
gen; fats,  however,  cannot  form  it.  In  severe  cases  of  diabetes 
it  is  therefore  usual  to  find  that  although  carbohydrate  foods 
are  entirely  withheld,  dextrose  continues  to  be  eliminated  in  the 
urine.  It  may  come  partly  from  the  protein  of  the  food  and 
partly  from  that  of  the  tissues. 

The  adjustment  between  the  rate  at  which  the  glycogen  of  the 
liver  becomes  converted  into  dextrose  and  the  percentage  of 
sugar  in  the  systemic  blood  is  effected  partly  through  the  nervous 
system  and  partly  by  means  of  substances  called  chemical  mes- 
sengers or  hormones  (see  p.  124)  secreted  into  the  blood  from  the 
ductless  glands,  such  as  the  pancreas  and  the  adrenals.  The 
very  first  symptoms  of  diabetes,  which  we  have  seen  to  consist  in 
an  excessive  postprandial  rise  in  the  systemic  blood-sugar  and  a 
consequent  glycosuria,  must  therefore  be  due  to  defects  in  one 
or  other  of  these  regulatory  mechanisms.  It  is  therefore  of  great 
interest  to  know  that  glycosuria  can  be  induced  in  the  lower 
animals  by  stimulation  of  the  nerves  of  the  liver  or  by  interfer- 
ing with  the  function  of  the  pancreas  or  the  adrenal  glands.  The 
^nerves  of  the  liver  may  be  stimulated  either  directly  or  through 
a  nerve  center  located  in  the  medulla  oblongata  (see  p.  246). 
Complete  removal  of  the  pancreas  is  followed  in  a  few  hours 
by  a  very  acute  form  of  diabetes,  which  is  invariably  fatal  in  a 
few  weeks,  whatever  the  treatment  may  be.    Injection  of  extract 


118  HUMAN   PHYSIOLOGY. 

of  the  adrenal  gland  (adrenalin)  causes  a  transient  hyperglycse- 
mia  and  glycosuria. 

These  laboratory  discoveries  have  in  their  turn  caused  clinical 
investigators  to  pay  close  attention  to  the  nature  of  the  causes 
of  dialetes.  It  has  been  found,  as  a  result,  that  oft-repeated 
overstimulation  of  the  nervous  system — nerve  strain,  as  it  is 
called — greatly  predisposes  to  this  disease.  For  example,  it  has 
been  found  th^t  a  considerable  proportion  of  students  who  un- 
derwent a  severe  examination  for  a  university  degree  had  sugar 
in  the  urine  which  was  passed  immediately  after  leaving  the 
examination  room.  Even  more  interesting  was  the  observation 
of  that  of  a  number  of  men  waiting  on  the  side  lines  as  reserves 
in  one  of  the  large  football  games,  about  one-half  of  them  passed 
sugar,  due  to  nervous  excitation  of  the  glycogenic  function.  Be- 
sides these  types  of  nerve  strain,  nervous  glycosuria  may  also  be 
brought  on  by  fright  and  terror.  This  has  perhaps  been  most 
definitely  shown  by  frightening  a  tom-cat  by  allowing  a  dog  to 
bark  at  it;  the  cat  shortly  afterward  passed  urine  containing 
much  sugar.  Now,  whereas  occasional  attacks  of  such  nervous 
glycosuria  are  harmless,  yet  their  repeated  occurrence  undoubt- 
edly weakens  the  ability  of  the  liver  to  control  properly  the  per- 
centage of  sugar  in  the  blood,  with  the  consequence  that  post- 
prandial hyperglycEemia  becomes  more  and  more  marked  and 
takes  longer  to  disappear,  so  that  there  comes  to  be  a  permanent 
increase  in  the  percentage  of  sugar  in  the  blood.  This  persistent 
excess  of  sugar  acts  as  a  poison  and  causes  deterioration  of  many 
of  the  tissues,  and  if  unchecked  will  lead  to  severe  diabetes.     - 

It  is  for  these  reasons  that  diabetes  is  relatively  common 
amongst  locomotive  engineers  and  ship  captains;  it  is  also 
said  to  be  distinctly  on  the  increase  amongst  business  men.  A 
most  important  element  in  the  treatment  of  diabetes  is  therefore 
removal  of  the  possible  causes  of  nerve  strain.  Rest  and  quiet 
and  freedom  from  worry,  coupled  with  removal  of  sufficient 
amounts  of  carbohydrates  from  the  diet  so  as  to  keep  the  urine 
free  of  sugar,  is  the  correct  treatment.  One  common  symptom 
of  diabetes  is  loosening  of  the  teeth.  When  this  is  observed  the 
urine  passed  an  hour  or  so  after  lunch  should  be  examined  for 


THE   METABOLISM   OF   INORGANIC   SALTS.  119 

sugar.  Properh^  conducted  treatment  Mall  often  cause  the  teeth 
to  tighten  up  again. 

A  very  common  cause  of  death  in  diabetes  is  coma,  which  is 
due  to  the  poisoning  of  the  animal  by  acid  substances  (oxy- 
butyric  acid)  resulting  from  the  imperfect  oxidation  of  fat 
(see  p.  116).  While  these  acid  substances  are  gradually  accumu- 
lating in  the  blood,  the  organism  attempts  to  neutralize  them  by 
diverting  ammonia  from  its  normal  course  into  urea  (see  p.  108)  ; 
hence  the  ammonia  content  in  the  urine  is  very  high  in  severe 
cases  of  diabetes.  Along  with  these  acids  and  ammonia,  acetone 
also  appears  in  the  urine  and  breath,  so  that  one  can  often  diag- 
nose a  severe  case  of  diabetes  by  the  smell  of  these  substances 
in  the  breath.  Diabetes  is  therefore  a  disease  which  the  dentist 
should  always  be  on  the  lookout  for. 

Metabolism  of  the  Inorganic  Salts. — Being  already  com- 
pletely oxidized,  inorganic  salts  cannot  yield  any  energy  during 
their  passage  through  the  animal  body  but  nevertheless  they  are 
essential  to  life.  They  are  used  not  only  for  the  building  up  of 
bones  and  teeth,  but  also  for  the  proper  carrying  out  of  the 
metabolic  processes.  In  this  respect  they  are  like  the  lubricant 
of  a  piece  of  machinery,  the  organic  foodstuffs  being  like  the  fuel. 

Their  indispensability  is  very  clearly  shown  by  the  fact  that 
animals  die  sooner  when  they  are  fed  on  food  from  which  all 
traces  of  inorganic  salts  have  been  extracted  than  when  they  are 
deprived  of  food  altogether.  This  result  shows  us  that  during 
the  metabolism  of  organic  foods  substances  must  be  produced 
which  act  as  poisons  in  the  absence  of  inorganic  salts.  Some  of 
these  poisonous  substances  are  no  doubt  acid  in  reaction  because 
life  can  be  prolonged  for  some  time  by  merely  adding  sodium 
carbonate  to  the  salt-free  food.  But  salts  not  having  any 
neutralizing  powers  are  also  necessary  to  keep  the  animal 
alive. 

The  chief  salts  which  we  take  with  our  food  are  the  chlorides, 
carbonates  and  organic  acid  salts  (e.  g.,  citrates,  tartarates,  etc.) 
of  sodium  and  potas.sium  and  of  calcium.  We  also  take  some  iron 
and  traces  of  iodine.  All  of  these  are  already  present  in  suffi- 
cient amount  in  the  ordinary  foodstuffs,  except  sodium  chloride, 


120  HUMAN   PHYSIOLOGY, 

or  common  salt.  This  we  must  add  to  our  food.  The  extent  to 
which  the  addition  of  common  salt  is  made  varies  very  strikingly 
according  to  the  nature  of  the  organic  food.  When  this  is 
mainly  vegetable  in  origin,  much  common  salt  is  required,  the 
reason  being  apparently  that  vegetables  contain  large  quantities 
of  potassium  salts  which  would  be  harmful  unless  a  proper  pro- 
portion of  sodium  is  also  taken.  The  demand  for  sodium  by 
herbivorous  animals  often  inclines  these  to  wander  for  hundreds 
of  miles  from  their  feeding  grounds  to  salt  licks.  Here  they  take 
enough  sodium  chloride  to  last  them  for  some  time.  The  carniv- 
orous animals  do  not  visit  salt  licks  unless  it  be  for  the  purpose 
of  preying  on  the  herbivorous  visitors.  The  salt  hunger  from 
which  they  suffer  compels  the  herbivora  to  go  to  the  salt  licks 
even  in  the  face  of  this  danger  of  destruction  by  the  carnivora. 
The  same  relationship  between  the  desire  for  salt  and  the  diet 
is  seen  in  man,  the  salt  consumption  per  capita  being  much 
greater  in  rural  than  in  urban  communities. 

Usually  enough  iron  is  taken  either  in  meats  or  in  certain  vege- 
tables, as  spinach.  The  body  is  very  careful  of  its  supply  of 
iron  (which  is  the  most  important  constituent  of  haemoglobin), 
but  if  it  loses  it  more  quickly  than  the  loss  can  be  made  good 
from  the  food,  anemia  results  and  it  becomes  necessary  to  pre- 
scribe iron  salts  as  medicine. 

Similarly  with  calcmm,  there  is  usually  enough  in  the  food 
even  of  growing  animals  to  meet  the  demands  which  bone  and 
teeth  formation  entails.  Rickets  is  not  usually  due  to  a  defi- 
ciency of  calcium  in  the  food,  but  to  a  depraved  condition  of  the 
general  nutrition,  making  it  impossible  for  the  available  calcium 
to  be  properly  used.  Good  food,  air  and  exercise,  rather  than 
drugs,  is  the  correct  treatment  for  rickets. 

Our  knowledge  of  just  what  each  particular  inorganic  salt  does 
in  the  metabolism  of  an  animal  is  not  yet  very  far  developed,  but 
some  most  important  discoveries  have  been  made  in  this  connec- 
tion during  recent  years.  Thus,  by  observing  the  isolated  beat- 
ing heart  of  the  frog  or  turtle  it  has  been  found  that  a  certain 
proportion  of  sodium,  calcium  and  potassium  salts  is  essential 
to  the  maintenance  of  a  proper  beat.     With    sodium    chloride 


VITAMINES.  121 

alone  the  beat  soon  stops,  with  excess  of  potassium  an  immediate 
paralysis  occurs,  and  with  excess  of  calcium  an  immediate  rigor 
or  permanent  contraction.  Analogous  results  are  obtained  with 
other  muscles.  Salts  in  certain  proportions  may  even  cause 
processes  of  cell  division  to  start  in  the  ova  of  some  of  the  lower 
animals.  In  other  words,  a  process  of  embryo  development 
which  is  usually  induced  by  impregnation  by  the  male 
elements  may  be  made  to  start  by  the  action  of  salts. 

Vitamines.— Another  class  of  bodies  called  vitamines  is  of 
great  importance  as  adjuncts  of  diet.  Without  them  metabolism 
becomes  upset,  and  serious  symptoms  make  their  appearance  with 
perhaps  death  as  the  .ultimate  result ;  and  this  happens  even  al- 
though the  protein,  fat,  carbohydrate  and  inorganic  salts  of  the 
diet  be  in  proper  proportion.  The  first  indication  of  the  import- 
ance of  vitamines  was  furnished  by  observations  on  a  disease 
called  Beri-Beri,  which  occurs  among  peoples  of  tropical  coun- 
tries, and  is  characterized  by  severe  neuralgic  pains,  muscular 
weakness  and  paralysis;  symptoms  which  are  due  to  inflamma- 
tion of  the  nerves  (neuritis).  It  was  noted  that  it  occurred  most 
frequently  in  the  case  of  people  whose  main  article  of  diet  was 
polished  rice,  but  was  infrequent  in  the  case  of  those  using  the 
unpolished  grain.  The  difference  between  these  two  grades  of 
rice  is  that  the  one  (the  unpolished)  still  contains  some  of  the 
brownish  husk ;  the  other  is  free  of  it.  This  observation  suggested 
the  experiment  of  adding  some  of  the  ground-up  rice  husks  to  the 
polished  rice  diet  of  those  suffering  from  the  disease,  with  the 
result  that  the  symptoms  soon  disappeared.  Moreover,  when 
unpolished  rice  was  supplied,  in  place  of  polished  rice,  to  natives 
among  whom  Beri-Beri  was  very  prevalent,  the  disease  disap- 
peared entirely.  Other  foodstuffs  contain  this  vitamine,  so  that 
Beri-Beri  does  not  occur  with  mixed  diets. 

In  order  to  learn  something  more  about  these  remarkable  sub- 
stances it  was  necessary  to  seek  for  some  animal  in  which  symp- 
toms similar  to  those  of  Beri-Beri  could  be  induced  by  feeding 
with  polished  rice.  Pigeons  were  found  most  suitable.  "When 
these  birds  are  kept  exclusively  on  such  a  diet,  they  develop  the 


122  HUMAN   PHYSIOLOGY. 

most  alarming  symptoms  of  neuritis  (paralysis,  weakness,  etc.), 
which  however  disappear  in  a  few  hours,  not  only  when  unpol- 
ished rice  or  rice  polishings  (or  husks)  are  given,  but  also  when 
meat,  or  beans,  or  a  small  piece  of  yeast  is  mixed  with  the  rice. 
Attempts  have  naturally  been  made  to  isolate  the  substance 
which  is  responsible  for  this  remarkable  action,  and  indeed  some 
success  can  already  be  reported.  For  example,  it  has  been  pos- 
sible to  separate  from  rice  polishings  and  from  yeast  small  traces 
of  crystalline  substances  having  a  most  powerful  action  in  pre- 
venting neuritis. 

Even  such  success  in  investigating  the  cause  of  Beri-Beri  in 
rice-feeders  would  scarcely  warrant  us  in  asserting  that  vita- 
mines  are  essential  constituents  of  our  own  varied  diets.  To  show 
that  they  are,  however,  has  been  no  very  difficult  task.  Thus,  it 
is  known  that  although  young  rats  thrive  admirably  on  milk  diet, 
they  fail  to  do  so  on  one  of  artificial  milk,  that  is,  of  milk  made 
in  the  laboratory  by  mixing  together,  in  proper  proportions,  the 
same  proteins,  fats,  carbohydrates  and  salts  that  occur  in  milk. 
In  this  chemical  mixture,  something  is  wanting  which  exists  only 
when  the  ingredients  of  milk  are  compounded  by  the  mammary 
glands.  The  addition  to  synthetic  milk  of  desiccated  milk  from 
which  most  of  the  proteins  had  been  removed  bestowed  on  it  full 
nutritive  value. 

The  practical  importance  of  this  observation  in  the  feeding  of 
infants,  we  need  not  insist  on.  Suffice  it  to  say  that  it  is  quite 
possible  that  prolonged  boiling  of  riiilk,  as  for  its  sterilization, 
may  deprive  it  of  vitamines  and  thus  render  the  child  liable  to 
such  diseases  as  rickets  and  infantile  scurvy,  or  at  least  interfere 
materially  with  its  proper  development  and  growth.  Among  the 
symptoms  thus  produced,  especially  in  the  case  of  infantile 
scurvy,  ulcers  may  develop  on  the  gums,  or  the  teeth  may  become 
loosened.  Change  of  diet  may  in  a  few  days  restore  perfect 
health,  or  even  the  addition  of  a  few  teaspoonfuls  of  orange  or 
lemon  juice  to  the  original  diet  may  suffice.  It  is  often  miracu- 
lous how  quickly  such  treatment  may  change  a  fretful,  pain- 
stricken  child  to  one  of  perfect  health  and  cheerfulness. 

Innumerable  other  examples  of  the  wonderful  influence  of 


VITAMINES.  123 

these  mysterious  vitamines  in  nutrition  might  be  cited.  The 
practical  point  to  bear  in  mind  is  that,  however  correctly  our 
diet  may  be  composed  with  regard  to  calorie  and  chemical  re- 
quirements, it  is  likely  to  be  unsuitable  unless  it  contains  a  cer- 
tain, though  perhaps  extremely  minute,  amount  of  the  drug-like 
substances  called  vitamines. 


CHAPTER  XIII. 
THE  DUCTLESS  GLANDS. 

Introductory. — ^We  have  no  more  than  touched  the  very 
fringe  of  the  subject  of  metabolism,  and  yet  we  have  learned 
enough  to  impress  us  with  the  fact  that  although  the  chemical 
processes  occurring  in  the  body  are  extremely  complicated,  they 
are  nevertheless  under  perfect  control.  We  must  now  learn 
something  regarding  the  nature  of  this  control. 

If  we  take  such  a  metabolic  process  as  that  which  carbohy- 
drates undergo,  we  should  expect  that  the  conditions  which  deter- 
mine whether  glycogen  shall  be  formed  or  broken  down  would 
be  chemical  in  nature.  "We  should  expect,  in  other  words,  that 
some  change  in  the  chemical  composition  of  the  blood — either  its 
reaction  or  the  amount  of  sugar  in  it,  or  the  appearance  in  it  of 
some  decomposition  product  of  sugar — would  determine  whether 
or  not  glycogen  should  be  mobilized  as  sugar.  In  muscular  work, 
for  example,  sugar  is  required  by  the  contracting  muscles,  and 
we  find  that  the  glycogen  stores  in  the  liver  become  very  quickly 
depleted  to  meet  the  demand.  The  question  is,  how  do  the  mus- 
cles transmit  their  requirements  to  the  liver  so  as  to  cause  this 
organ  to  mobilize  the  dextrose  ?  Our  natural  assumption  would 
be  that  the  active  muscles  cause  some  change  to  occur  in  the 
blood  and  that  it  is  this  change  which  excites  the  liver  cells. 
Such  a  control  of  the  metabolic  activities  of  one  tissue  by  prod- 
ucts of  the  activity  of  another,  transmitted  between  them  by 
way  of  the  blood,  is  known  as  hormone  control.  We  have  already 
become  acquainted  with  it  in  connection  with  the  control  of  cer- 
tain of  the  digestive  glands,  particularly  the  pancreas  (see 
p.  72),  and  it  is  no  doubt  very  largely  by  such  a  mechanism 
that  a  given  metabolic  process  becomes  active  or  supressed,  as 
occasion  demands. 

The  hormones  in  such  cases  are  in  part  the  intermediary  prod- 
ucts of  metabolism,  but  besides  these  hormones  others  must  exist 

124 


THE  THYROID  GLAND.  125 

to  call  forth  or  regulate  the  activities  of  tissues  which  are  not 
immediately  concerned  in  general  metabolism  but  rather  with 
special  processes,  such  as  the  excitability  of  the  nervous  system 
(e.  g.,  adrenalin),  the  behavior  of  the  reproductive  glands  (e.  g., 
in  the  secretion  of  milk),  the  growth  of  certain  tissues  (e.  g.,  of 
subcutaneous  tissues,  of  hairs)  or  the  atrophy  of  others,  (e.  g., 
of  the  uterus  after  pregnancy  is  terminated).  For  such  hor- 
mones, special  manufacturing  centres  are  provided  in  the  duct- 
less glands.  The  thyroid  and  thymus  glands  in  the  neck,  the 
pituitary  in  the  brain,  the  spleen  and  adrenal  glands  in  the  ab- 
domen are  good  examples.  None  of  these  has  any  duct,  but  they 
discharge  the  products  of  their  activity — internal  secretion — 
into  the  blood  stream,  by  which  it  is  carried  to  the  tissue  or  organ 
on  which  it  acts.  Internal  secretions  may  also  be  produced  by 
certain  cells  of  the  digestive  glands,  as,  for  example,  the  so-called 
Isles  of  Langerhans  of  the  pancreas  (see  p.  72),  and  likewise 
there  are  certain  organs,  such  as  the  ovaries  and  testes,  whose 
main  functions  are  of  a  special  nature,  but  which  also  possess 
the  power  of  producing  very  powerful  internal  secretions. 

We  shall  confine  our  attentions  for  the  present,  however,  to 
the  strictly  ductless  glands.  Their  function  is  ascertained  ex- 
perimentally either  by  removing  the  gland  by  operation  or  by  in- 
jecting an  extract  of  it  and  then  observing  the  behavior  of  the 
animal.  Much  can  also  be  learned  by  observing  patients  in  whom 
the  gland  is  diseased. 

The  Thyroid  and  Parathyroid  Glands. — The  thyroid  gland 
consists  of  two  oval  lobes  situated  one  on  either  side  of  the 
trachea  just  below  the  larynx  or  voice  box,  and  connected  to- 
gether over  the  trachea  by  an  isthmus  of  thyroid  tissue.  Em- 
bedded in  the  substance  of  each  lobe  of  the  gland  on  the  poste- 
rior surface  are  the  two  very  small  paratJiyroid  glands.  Minute 
examination  shows  the  thyroid  glands  to  be  composed  of  vesicles 
lined  by  low  columnar  epithelium  and  filled  with  a  clear  glossy 
substance  called  colloid.  The  parathyroids  have  an  entirely  dif- 
ferent structure,  being  composed  of  elongated  groups  of  poly- 
hedral cells  with  no  colloid  material. 

The  functions  of  the  two  glands  are  probably  essentially  dif- 


126  HUMAN   PHYSIOLOGY. 

ferent,  the  thyroid  having  to  do  with  the  general  nutrition  of  the 
animal,  and  the  parathyroid  with  the  condition  of  the  nervous 
system.  They  lie  so  close  together,  however,  that  it  is  very  diffi- 
cult to  study  their  separate  functions.  The  importance  of  the 
glands  is  indicated  by  the  relatively  large  blood  supply. 


Fig.  10. — Cretin,  19  years  old.  The  treatment  with  thyroid  extract  was 
started  too  late  to  be  of  benefit.      (Patient  of  Dr.  S.  J.  V^ebster.) 

When  the  thyroid  is  not  properly  developed  in  children,  the 
condition  is  known  as  cretinism  (Fig.  10).  The  child  fails  to 
grow  in  height,  although  its  bones  may  thicken.  The  cranial 
bones  soon  fuse  together,  so  that  the  growth  of  the  brain  is  hin- 


THE  THYROID   GLAND. 


127 


dered  and  the  mental  powers  fail  to  develop.  The  child  becomes 
idiotic,  and  although  it  may  live  for  years,  it  will  remain,  even 
at  thirty  years  of  age,  a  stunted,  pot-bellied,  ugly  creature  with 
the  intelligence  of  an  infant.  The  cause  of  this  failure  to  de- 
velop is  undoubtedly  bound  up  in  some  way  with  the  deficiency 
of  the  thyroid,  for  if  the  cretin  be  given  the  extract  of  this  gland, 
its  condition  will  immediately  improve,  and  indeed,  if  taken 
early  enough,  it  may  quickly  make  up  for  lost  time  and  grow 
both  physically  and  mentally  as  it  ought  to. 

Atrophy  of  the  thyroid  gland  in  older  persons  causes  myxoe- 
dema.     (Fig.  11).     The  symptoms  of  tliis  are  very  characteris- 


A.  B. 

Fig.   11. — A,  Ca.se  of  myxoedema  ;  B,  Samf  after  seven   months'   treatment. 
(Tigerstedt.) 

tic,  being  most  commonly  seen  in  women.  The  skin  is  dry 
and  often  of  a  yellowish  color,  the  hair  falls  out,  the  subcutaneous 
tissues  grow  excessively,  so  that  the  hands,  the  feet  and  the  face 
become  large  and  puffy,  and  the  speech  indistinct,  because  of  the 
thickening  of  the  lips.  The  metabolism  also  becomes  very  slug- 
gi.sh,  so  that  the  intake  of  food  and  the  excretion  of  nitrogen  in 
the  urine  become  diminished,  and  the  temperature  subnormal.    If 


128  HUMAN   PHYSIOLOGY. 

unchecked,  mental  symptoms  become  apparent,  first  of  all,  a 
dulling  of  the  intellect  with  sleepiness  and  lethargy,  and  later, 
muscular  twitchings  and  tremors.  Just  as  in  cretinism,  so  in 
myxoedema,  administration  of  thyroid  extract  causes  these  symp- 
toms to  disappear,  so  that  in  a  month  or  so  the  patient  may  have 
returned  to  his  or  her  normal  condition,  to  maintain  which,  how- 
ever, the  thyroid  extract  must  continue  to  be  given. 

When  the  gland  is  removed  surgically,  either  in  lower  animals 
or  in  man,  very  acute  symptoms  ending  in  death  usually  super- 
vene. These  include  a  peculiar  form  of  muscular  tremor  called 
tetany,  passing  into  actual  convulsions,  which,  by  involving  the 
respiratory  muscles,  ultimately  cause  dyspnoea  and  death.  It 
is,  however,  probable  that  these  nervous  symptoms  are  due  to  the 
unavoidable  removal  of  the  parathyroid  glands.  The  tetany  is 
removed  by  giving  calcium  salts.  These  conditions  associated 
with  deficiency  of  the  thyroid  are  grouped  together  as  hypothy- 
roidism. 

Even  in  healthy  individuals  thyroid  extract  taken  by  mouth 
excites  a  more  active  metabolism,  and  may  cause  increased  heart 
activity.  One  result  of  this  increased  metabolism  is  disappear- 
ance of  subcutaneous  fat  and  increased  appetite,  thus  rendering 
the  administration  of  moderate  doses  of  thyroid  extract  a  not 
uncommon  method  of  treatment  for  obesity.  Such  treatment 
should  never  be  attempted  except  under  the  control  of  a  physi- 
cian, for  it  is  very  easy  to  take  too  much  of  the  extract  and  cause 
palpitation  and  nervous  excitement. 

"When  the  thyroid  (and  parathyroid)  glands  become  excess- 
ively active  in  man,  the  condition  is  called  hyperthyroidism,  and 
the  symptoms  are  very  like  those  above  described  as  produced 
by  taking  thyroid  extract.  To  be  exact,  they  are  palpitation; 
wasting  of  the  muscles  and  consequent  weakness,  extreme  ner- 
vousness and  protrusion  of  the  eyeballs.  On  account  of  this  last 
mentioned  symptom  the  condition  is  usually  called  exophthalmic 
goitre.  This  acute  and  often  fatal  disease  is  to  be  distinguished 
from  chronic  goitre,  in  which  there  are  very  few  general  symp- 
toms, but  great  enlargement  of  the  thyroid  gland,  indeed  an  en- 
largement which  may  be  so  pronounced  as  practically  to  obliter- 


THE   ADRENAL   GLANDS  129 

ate  the  neck  and  sometimes  so  compress  the  trachea  as  to  inter- 
fere with  breathing.  The  cases  of  chronic  goitre  occur  in  the 
same  districts  in  which  the  exophthalmic  variety  is  common,  these 
being,  in  this  country,  the  shores  of  the  great  inland  lakes  and 
the  I'iver  valleys,  but  not  in  districts  bordering  on  the  sea.  They 
are  also  common  in  certain  districts  in  Switzerland  and  Eng- 
land.  It  is  of  interest  that  in  the  lake  and  river  districts  in 
this  country  the  thyroids  of  over  ninety  per  cent  of  all  dogs  are 
more  or  less  hypertrophied. 

The  above  remarkable  influence  of  the  thyroids  on  metabolism 
is  in  some  way  dependent  upon  the  colloid  material  which  fills 
the  vesicles  of  the  gland.  This  colloid  contains  a  peculiar  sub- 
stance called  iodothyrin,  because  it  contains  iodine,  an  element 
wliifli  is  not  found  present  in  any  other  part  of  the  animal  body. 

The  Adrenal  Glands. — As  their  name  signifies,  these  are  situ- 
ated one  on  either  side  just  above  the  kidneys.  Each  gland  is 
yellowish  in  color,  and  is  seen  on  microscopic  examination  to  be 
composed  of  a  medullary  and  a  cortical  portion.  The  medulla 
consists  of  irregular  collections  of  cells  containing  granules 
which  stain  deeply  brown  with  chromic  acid  and  are  therefore 
called  chromophile  granules.  Similar  chromophile  granules  may 
exist  in  other  parts  of  tlie  body.  The  great  splanchnic  nerve, 
which  it  will  be  remembered  arises  from  the  sympathetic  chain 
in  the  thorax  (see  p.  278),  makes  very  intimate  connection  with 
the  adrenal  medulla,  for  which  reason  and  because  of  the  fact 
that  it  is  developed  from  the  same  embryonic  tissue  as  the  sym- 
pathetic system  of  nerves,  the  medulla  of  the  adrenal  gland  is 
believed  to  be  closely  bound  up  with  the  functions  of  the  sympa- 
thetic nervous  system.  The  cortex  is  composed  of  rows  of  col-- 
unjnar  cells  which  do  not  contain  chromophile  granules.  Small 
though  they  be,  the  adrenal  glands  are  essential  to  life,  for  their 
removal  causes  extreme  muscular  weakness  and  a  fall  in  blood 
pressure  followed  by  death  within  twenty-four  hours.  When 
they  are  the  seat  of  disease  (tuberculous),  symptoms  of  extreme 
muscular  prostration,  accompanied  by  vomiting  and  a  peculiar 
bronzing  of  the  skin,  s^;t  in  and  grow  steadily  worse  until  at  last 
the  patient  succumbs.    This  is  called  Addison's  disease. 


130  HUMAN   PHYSIOLOGY. 

The  most  striking  proof  of  their  importance  is  obtained  by  in- 
jecting an  extract  of  the  medulla  of  the  adrenal  gland  into  a 
vein.  It  causes  an  immediate  rise  in  blood  pressure,  which  is 
more  or  less  proportional  to  the  strength  of  the  extract.  The 
rise  is  accompanied  by  a  slowing  of  the  heart,  due  to  the  reflex 
stimulation  *of  the  vagus  centre  excited  by  the  rising  blood  pres- 
sure. When  this  reflex  slowing  is  rendered  impossible  by  cutting 
the  vagi,  the  rise  in  blood  pressure  following  the  injection  may 
be  enormous.  The  active  substance  in  the  extract  is  called  adren- 
alin, suprarenin,  adrenin  or  epinepJirin.  It  is  a  comparatively 
simple  chemical  body,  having  the  formula : 


(HO)C 
(HO)C 


CH 

C— CH  ( OH )  CH,— NHCH, 

ICH 


CH 

and  existing  in  two  varieties  which  differ  from  one  another  ac- 
cording to  the  direction  toward  which  the  plane  of  polarized  light 
is  rotated.  The  variety  rotating  to  the  left  is,  by  many  times, 
stronger  in  its  physiological  actions  than  that  which  rotates  to 
the  right.  The  discovery  of  its  chemical  structure  has  made  it 
possible  for  chemists  to  prepare  suprarenin  synthetically,  and 
also  to  prepare  a  series  of  related  substances  having  less  marked 
though  similar  properties.  These  are  closely  related  to  certain 
of  the  bodies  which  appear  during  the  putrefaction  of  meat. 

By  careful  studies  of  the  action  of  the  suprarenin,  or  related 
substances,  it  has  been  found  that  the  rise  in  blood  pressure,  above 
referred  to,  is  due  to  stimulation  of  the  muscle  fibers  in  the  walls 
of  the  blood  vessels.  It  is  on  this  account  that  a  weak  solution  of 
suprarenin  is  used  to  stop  haemorrhage,  as  after  removing  polypi 
from  the  nose,  or  in  bleeding  from  the  gums,  as  after  tooth  ex- 
traction. The  muscle  of  arteries  is  by  no  means  the  only  struc- 
ture on  which  adrenalin  acts ;  indeed  it  stimulates  every  structure 
which  is  capable  of  being  stimulated  by  the  sympathetic  nervous 
system  (see  p.  277).     Thus,  it  causes  the  pupil  to  dilate,  saliva 


THE  PITUITARY  GLAND. 


131 


to  be  secreted  (p.  41),  the  movements  of  the  intestine  to  be  in- 
hibited (p.  79),  whereas  it  has  no  action  on  the  blood  vessels 
of  the  lungs  or  brain,  which  do  not  possess  vasomotor  nerves. 
This  similarity  between  the  results  which  follow  suprarenin  in- 
jection and  stimulation  of  the  sympathetic  system  is  particularly 
significant  when  we  call  to  mind  the  fact  that  the  medulla  of  the 
adrenal  gland  is  developed  from  the  same  embryonic  tissue  as 
the  sympathetic  system.  The  clotting  power  of  the  blood  is 
diminished  after  injections  of  suprarenin. 

The  Pituitary  Gland. — This  occupies  the  Sella  Turcica  of  the 
base  of  the  cranium  and  is  composed  of  three  portions  or  lobes. 
The  anterior  lobe  consists  of  large  epithelial  cells  and  is  really 
an  isolated  outgrowth  from  the  epiblast  of  the  upper  end  of  the 
alimentary  canal.  Its  complete  excision  causes  death  in  a  few 
days,  biit  if  only  a  part  is  removed,  a  condition  called  Jiypo- 
pituitarism  develops,  of  which  adiposity  and  sexual  impotence, 
are  the  main  symptoms.  When  this  lobe  becomes  excessively 
active  in  man  (because  of  hypertrophy),  it  causes  a  peculiar 
gro\\i;h  of  the  bones,  particularly  of  the  lower  jaw,  thus  making 
the  person  look  as  if  he  were  very  powerful.  This  disease  is 
called  acromegaly  (Fig.  12),  and  besides  the  changes  in  the 
bones,  there  is  frequently  considerable  metabolic  disturbance, 
causing  a  mild  form  of  diabetes.  When  the  hypertrophy  of  the 
anterior  lobe  occurs  in  youth,  most  of  the  bones  of  the  body  may 
be  affected,  thus  causing  the  condition  known  as  giantism. 

The  intermediary  lobe  is  also  composed  of  columns  of  epithe- 
lial cells,  but  there  is  often  some  colloidal  material  between  the 
columns.  This  colloid  differs  from  tliat  of  the  thyroid  in  con- 
taining no  iodine. 

The  posterior  lobe  is  really  a  downgrowth  from  the  brain,  and 
is  composed  of  neuroglia  mixed  with  some  of  the  epithelial  cells 
of  the  intermediary  lobe.  This  lobe"  can  be  excised  without  caus- 
ing any  evident  change  in  the  animal,  but  nevertheless  it  must 
have  some  important  functions  to  perform,  because  extracts  of  it, 
when  injected  intravenously,  have  very  pronounced  effects,  viz. : 
(1)  a  rise  in  blood  pressure;  (2)  a  very  striking  diuretic  action 
(i.  e.,  causes  urine  to  be  excreted)  ;  (3)  secretion  of  milk.    The 


132 


HUMAN  PHYSIOLOGY, 


active  principle  of  these  extracts  has  not  as  yet  been  isolated, 
although  the  extracts  can  be  considerably  concentrated,  thus 
yielding  the  trade  preparation  called  pihiitrin. 

It  is  particularly  interesting  to  note  that  although  the  anterior 
lobe  does  not  yield  any  active  extract,  yet  its  excision  is  fatal. 
On  the  other  hand,  the  posterior  lobe  can  be  removed  with  im- 
punity, although  extracts  of  it  have  profound  physiological 
effects  when  they  are  injected  into  normal  animals. 


A. 


B. 


Fig.  12. — A,  To  show  the  appearance  before  the  onset  of  acromegalic  symp- 
toms:  B,  The  appearance  after  seventeen  years  of  the  disease.  (Aftei 
Campbell   Geddes. ) 


The  Spleen. — Notwithstanding  the  fact  that  this  is  the  larg- 
est of  the  ductless  glands,  it  is  the  one  whose  functions  are  the 
least  well  understood.  It  can  be  excised  without  causing  any 
evident  disturbance,  and  extracts  of  it  when  injected  intraven- 
ously do  not  have  any  characteristic  effects.  It  becomes  very 
much  enlarged  in  certain  diseases,  namely:  (1)  in  leucocythe- 
mia,  a  form  of  anaemia,  which  is  characterized  by  a  great  increase , 
in  the  leucocytes  of  the  blood  (see  p.  145)  ;  (2)  in  typhoid  fever 
(enteric  fever)  ;   (3)   in  malaria.     It  becomes  contracted  after 


THE  THYMUS  GLAND.  133 

taking  quinine.  Under  the  microscope  it  is  seen  to  be  composed 
of  a  sponge  of  fibrous  tissue,  the  spaces  being  filled  with  blood, 
which  flows  freely  into  them  from  arterioles  in  whose  walls 
Ijonphoid  tissue  is  abundant.  Here  and  there,  this  lymphoid 
tissue  becomes  collected  in  nodules,  which  are  large  enough  to 
be  seen  by  the  naked  eye  and  are  called  Malpighian  corpuscles. 

In  the  blood  of  the  spleen,  partly  broken  down  erythrocytes 
are  often  visible.  Sometimes,  also,  cells  like  those  found  in  red 
bone  marrow  and  having  to  do  with  the  manufacture  of  new  red 
corpuscles  make  their  appearance. 

Taking  all  these  facts  together,  it  is  believed  that  the  spleen 
has  the  following  functions:  (1)  manufacture  of  leucocytes; 
(2)  manufacture  of  erythrocytes;  (3)  destruction  of  erythro- 
cytes; (4)   removal  from  the  blood  of  certain  poisons. 

The  Thymus  Gland. — The  thymus  gland,  situated  at  the  root 
of  the  neck,  is  quite  large  at  birth,  but  its  size  gradually  dimin- 
ishes as  the  animal  grows.  By  the  time  that  puberty  is  reached, 
it  has  almost  disappeared.  It  is  composed  of  peculiarly  arranged 
lymphoid  tissue,  having  nests  of  epithelial  cells  embedded  in  it. 
It  seems  to  bear  some  relationship  to  the  generative  glands,  for 
its  removal  in  j^oung  male  animals  hastens  the  growth  of  the 
testes. 


CHAPTER  XIV. 
ANIMAL  HEAT  AND  FEVER. 

In  considering  the  problem  of  animal  heat,  it  is  essential  to 
bear  clearly  in  mind  the  distinction  between  amount  and  inten- 
sity of  heat.  The  former  is  measured  in  calories  (see  p.  84), 
and  the  latter  in  degrees  of  temperature.  To  measure  the  tem- 
perature of  a  man  a  maximal  thermometer  with  the  Fahrenheit 
or  Centigrade  scale  is  placed  in  some  protected  part  of  the  body, 
as  the  mouth,  the  axilla  or  the  rectum.  It  is  found  by  such  meas- 
urement that  the  temperature  varies  according  to  the  site  of  ob- 
servation and  the  time  of  day.  It  varies  between  36.0°  C. 
(96.8°  F.)  and  37.8°  C.  (100.0°  F.)  in  the  rectum ;  between  36.3° 
C.  (97.3°  F.)  and  37.5°  C.  (99.5°  F.)  in  the  axilla;  and  between 
36.°  C.  (96.8°  F.)  and  37.25°  C.  (99.3°  F.)  in  the  mouth.  These 
variations  indicate  that  the  temperature  is  higher  in  the  deeper 
than  in  the  superficial  parts  of  the  body ;  in  other  words,  that 
the  visceral  blood  is  warmer  than  that  of  the  surface  of  the  body. 
The  variations  of  temperature  due  to  the  time  of  day  are  most 
evident  when  it  is  taken  in  the  rectum,  and  they  amount  in  health 
to  a  little  over  1°  C.  or  a  little  below  2°  F.,  the  highest  tempera- 
ture occurring  about  3  p.  m.,  and  the  lowest  about  3  a.  m.  This 
is  called  the  diurnal  variation  and  it  may  become  much  greater 
in  febrile  diseases. 

Animals  whose  temperature  behaves  as  above  described  are 
called  ivarm-hlooded  in  contrast  to  those  animals,  called  cold- 
blooded, in  whom  it  is  only  a  degree  or  two  above  that  of  the 
air,  with  which  it  runs  parallel.  Such  animals  include  fishes, 
amphibians,  snakes,  etc.  Between  the  cold  and  the  warm-blooded 
animals  is  a  group  in  which  the  animal  is  warm-blooded  in  sum- 
mer and  cold-blooded  in  winter.  These  are  the  hihernating  ani- 
mals, such  as  the  hedgehog,  the  marmot,  the  bat,  etc.  In  this 
connection  it  is  interesting  to  note  that  the  human  infant  be- 

134 


ANIMAL    HEAT    AND    FEVER.  135 

haves  more  or  less  like  a  cold-blooded  animal  for  some  time  im- 
mediately following  birth,  during  which  period  it  must  there- 
fore be  carefully  protected  from  cooling,  for,  if  its  temperature 
be  allowed  to  fall  to  any  considerable  extent,  it  is  not  likely  to 
survive.  It  takes  several  months  before  the  heat  regulating 
mechanism  becomes  so  developed  that  the  infant  can  withstand 
any  considerable  degree  of  cold. 

Factors  Concerned  in  Maintaining  the  Body  Temperature. — 
The  body  temperature  is  a  balance  between  heat  production  and 
heat  loss.  Heat  is  produced  by  combustion  of  the  organic  food- 
stuffs in  the  muscles,  the  amount  which  each  foodstuff  thus  pro- 
duces being  the  same  as  when  it  is  burned  outside  the  body, 
except  in  the  ease  of  protein,  when  allowance  must  be  made  for 
the  incomplete  combustion  of  this  substance  in  the  animal  body 
(see  p.  85).  The  muscles  are  therefore  the  furnaces  of  the  ani- 
mal body,  the  fuel  being  the  organic  foodstuffs.  Heat  is  lost 
from  the  body  mainly  from  the  skin,  but  partly  also  from  the 
lungs  and  in  excreta.  Heat  loss  from  the  skin  is  brought  about 
by  the  utilization  of  several  physical  processes,  namely:  (1)  by 
conduction  along  objects  which  are  in  contact  with  the  skin  or 
through  the  air;  (2)  by  convection,  that  is,  by  being  carried 
away  in  currents  of  air  which  move  about  the  body;  (3)  by  radi- 
ation; (4)  by  evaporation  of  sweat.  This  last  is  the  means  by 
which  most  heat  can  be  lost,  because  it  takes  a  large  amount  of 
latent  heat  to  vaporize  the  sweat  (see  p.  20). 

Heat  loss  from  the  lungs  is  mainly  due  to  vaporization  of 
water,  with  which  the  expired  air  is  saturated.  A  small  amount 
is  also  absorbed  in  warming  the  air  itself.  The  heat  lost  in  the 
urine  and  fa»cos  is  almost  negligible. 

The  Regulation  of  the  Body  Temperature. — It  is  plain  that 
a  very  sensitive  regulatory  mechanism  must  exist  in  order  that 
the  production  and  loss  of  heat  may  be  so  adjusted  as  to  keep  the 
body  temperature  practically  constant.  When  heat  loss  becomes 
excessive,  then  must  heat  production  be  increased  to  maintain  the 
balance,  and  vice  versa  when  heat  loss  is  slight.  The  conditions 
are  to  a  certain  extent  comparable  with  those  obtaining  in  a 
house  heated  by  a  furnace  and  radiators  and  provided  with  a 


136  HUMAN  PHYSIOLOGY. 

thermo-regulator,  which,  being  activated  by  the  temperature  of 
the  "rooms,  acts  on  the  furnace  so  as  to  raise  or  lower  its  rate  of 
combustion. 

In  the  animal  body  the  thermo-regulator  is  the  nervous  sys- 
tem. "Whenever  the  temperature  of  the  blood  changes  from  the 
normal,  a  nerve  center  called  the  thermogenic  becomes  acted  on 
with  the  result  that  it  transmits  impulses  to  the  muscles,  which, 
by  increasing  or  diminishing  their  tone  (see  p.  253),  cause  a 
greater  or  a  less  heat-production.  But  the  center  does  more  than 
the  thermo-regulator  of  a  house,  for  it  controls  the  agencies  of 
heat-loss.  Thus,  when  the  blood  temperature  tends  to  rise,  the 
thermogenic  center  causes  more  heat  to  be  lost  from  the  skin  and 
lungs  in  the  following  ways:  (1)  It  acts  on  the  blood  vessels  of 
the  skin,  causing  them  to  dilate  so  that  more  blood  is  brought  to 
the  surface  of  the  body  to  be  cooled  off.  (2)  It  excites  the  sweat 
glands,  so  that  more  heat  has  to  be  utilized  to  evaporate  the  sweat. 
(3)  It  quickens  the  respirations,  so  that  more  air  has  to  be 
warmed  and  saturated  with  moisture.  The  degree  to  which  these 
cooling  processes  are  used  varies  in  different  animals.  Thus  in 
the  dog,  since  there  are  no  sweat  glands  over  the  surface  of  the 
body  (they  are  confined  to  the  pads  of  the  paws),  increase  in  the 
respiration  is  the  chief  method  of  cooling,  hence  the  panting 
on  warm  days. 

In  the  ease  of  man,  civilization  has  stepped  in  to  assist  the 
reflex  control  of  heat  loss,  as  by  the  choice  of  clothing  and  the 
artificial  heating  of  rooms.  Desirable  though  this  voluntary 
control  of  heat-loss  from  the  body  may  be,  there  can  be  little 
doubt  that  it  is  often  overdone  to  the  detriment  of  good  health. 
Living  in  overheated  rooms  during  the  cooler  months  of  the 
year  suppresses  to  a  very  low  degree  the  heat  loss  from  the  body 
and  thereby  lowers  the  tone  and 'heat  production  of  the  museular 
system.  The  food  is  thereby  incompletely  metabolized  and  is 
stored  away  as  fat ;  the  superficial  capillaries  are  constricted  and 
the  skin  becomes  bloodless.  But  it  is  not  looks  alone  that  suffer, 
but  health  as  well,  for,  by  having  so  little  to  do,  the  heat-regulat- 
ing mechanism  gets  out  of  gear  so  that  when  it  is  required  to 
act,  as  when  the  person  goes  outside,  it  may  not  do  so  promptly 


ANIMAL  HEAT  AND  FEVER.  137 

enough,  with  the  result  that  the  body  temperature  falls  some- 
what, and  catarrhs,  etc.,  are  the  result.  There  can  be  little  doubt 
that  much  of  the  benefit  of  open-air  sleeping  is  due  to  the  con- 
stant stimulation  of  the  metabolic  processes  which  it  causes. 

The  importance  of  the  evaporation  of  sweat  in  bringing  about 
loss  of  heat  in  man  partly  explains  why  climate  should  have  so 
important  an  influence  on  his  well-being.  It  is  not  so  much  the 
temperature  of  the  air,  as  its  relative  humidity,  that  is  of  impor- 
tance ;  that  is,  the  degree,  expressed  in  percentage,  to  which  the 
air  is  saturated  with  moisture  at  the  temperature  of  observation. 
Thus,  a  relative  humidity  of  75  per  cent  at  15°  C.  means  that 
the  air  contains  75  per  cent  of  the  total  amount  of  moisture  which 
it  would  contain  if  it  were  saturated  with  moisture  at  a  tempera- 
ture of  15°  C.  A  high  relative  humidity  at  a  high  temperature 
makes  it  impossible  for  much  sweat  to  be  evaporated,  with  the 
result  that  the  body  cannot  cool  properly,  and  the  body  tempera- 
ture is  likely  to  rise  unless  muscular  activity  be  reduced  to  a 
minimum.  This  explains  why  it  is  impossible  to  do  much  muscu- 
lar work  in  hot  humid  atmospheres.  On  the  other  hand,  if  the 
relative  humidity  is  low,  the  temperature  may  rise  to  an  extraor- 
dinary degree  (even  above  that  of  the  body  itself)  without  caus- 
ing fever,  provided  always  that  the  body  is  not  so  covered  with 
clothing  that  evaporation  of  sweat  is  impossible. 

At  low  temperatures  of  the  air,  relative  humidity  has  an  effect 
which  is  exactly  opposite  to  that  which  it  has  at  high  tempera- 
tures, for  now  it  affects,  not  the  evaporation  of  sweat,  but  the 
heat  conductivity  of  the  air  itself.  Cold  moist  air  conducts 
away  heat  much  more  rapidly  than  cold  dry  air.  Hence,  a  tem- 
perature many  degrees  below  zero  on  the  dry  plains  of  the  West 
may  be  much  more  tolerable  to  man  than  a  much  higher  tem- 
perature along  the  shores  of  the  Great  Lakes. 

Fever. — Any  rise  of  temperature  above  the  normal  limits 
constitutes  fever.  When  of  slight  degree,  as  it  is  in  many  semi- 
acute  diseases,  its  detection  demands  frequent  observation,  so 
as  to  allow  for  the  normal  diurnal  variation  of  the  body  tempera- 
ture. For  example,  if  the  temperature  were  recorded  in  the 
morning  in  such  a  patient,  a  slight  degree  of  fever  might  quite 


138  HUMAN   PHYSIOLOGY. 

easily  be  missed,  because  at  this  time  the  normal  temperature  is 
low.  In  acute  infectious  diseases,  the  afternoon  temperature 
may  rise  to  106°  F.  or  41°  C,  or  even  above  this,  without  prov- 
ing fatal.  A  temperature  of  113°  F.  or  45°  C.  has  been  observed, 
but  lasting  for  only  a  short  time.  Fever  is  always  higher  in  in- 
fants and  young  children  than  in  adults. 

As  to  tJie  causes  of  fever,  two  possibilities  exist :  either  (1)  that 
heat  production  has  been  increased,  or  (2)  that  heat  loss  has 
been  diminished,  or,  of  course,  both  factors  may  operate  simul- 
taneously. To  go  into  this  unsolved  problem  is  unnecessary  here ; 
suffice  it  to  say  that  there  can  be  no  doubt  that  disturbance  in 
the  thermogenic  center  is  the  underlying  cause  of  fever,  and 
that  it  is  the  avenues  of  heat  loss  by  the  skin  rather  than  the 
sources  of  heat  supply  in  the  muscles  that  are  first  of  all  acted 
on.  The  cold  sensation  down  the  back,  the  shivering,  the  goose 
skin,  are  the  familiar  initial  symptoms  of  fever,  and  when  the 
fever  comes  to  an  end,  excessive  sweating  sets  in  and  this,  in  part 
at  least,  explains  the  fall  in  temperature.  Increased  combustion 
in  the  muscles  no  doubt  occurs  during  the  height  of  the  fever 
and  accounts  for  the  great  wasting,  but  that  this  is  not  the  only 
cause  of  the  rise  in  temperature  is  evidenced  by  the  fact  that 
severe  muscular  exercise  does,  not  in  itself  cause  fever,  even 
although  there  may  be  much  more  combustion  going  on  in  the 
body  (see  p.  88). 

Certain  drugs  called  antipyretics  lower  the  temperature  in 
fever.  The  most  important  of  these  are  acetanilide,  salicylates 
(aspirin),  phenacetin,  and  quinine.  The  first  three  mentioned 
act  on  the  thermogenic  center,  whereas  quinine  seems  to  act 
directly  on  the  combustion  processes  in  the  muscles.  The  body 
temperature  is  raised  by  cocaine  and  by  the  toxic  products  of 
bacterial  growth.  Even  cultures  which  have  been  attenuated  by 
keeping  them  for  some  time  at  high  temperatures  have  this 
effect,  and  it  is  believed  by  many  that  fever  is  of  the  nature  of 
a  protective  mechanism  to  destroy  or  attenuate  the  invading 
bacteria.  There  is  bacteriological  as  well  as  clinical  support  for 
this  view,  thus,  certain  pathogenic  organisms  (such  as  the  strep- 
tococcus of  erysipelas)  cannot  live  at  a  temperature  above  41°  C, 


ANIMAL    HEAT    AND    FEVER.  139 

and  cholera  patients  are  much  more  likely  to  survive  if  the  dis- 
ease be  accompanied  by  a  moderate  degree  of  fever. 

Heat  stroke,  or  sun  stroke,  is  due  to  an  increase  in  body  tem- 
perature that  is  above  the  limits  of  safety.  When  sweating  and 
the  other  processes  by  which  heat  is  lost  from  the  body  are  act- 
ing properly  it  is  remarkable  how  high  an  air  temperature  may 
be  borne  without  danger;  for  example,  in  dry  air  a  man  can  sit 
for  some  minutes  in  an  oven  at  100 °C.  while  his  dinner  cooks 
beside  him  (Leonard  Hill).  But  if  anything  should  interfere 
with  heat  loss,  or  if  heat  production  be  excessive,  as  during  mus- 
cular exercise,  there  is  always  danger  of  heat  stroke.  Free  move- 
ment of  the  air  is  probably  the  most  important  way  for  safe- 
guarding against  deficient  heat  loss.  It  is  almost  certainly  on 
account  of  the  absence  of  such  air  movement,  coupled  with  a 
high  relative  humidity,  that  discomfort  is  experienced  in  hot, 
stuff}^  atmospheres,  for  the  faulty  heat  loss  causes  a  slight  rise 
in  body  temperature.  This  slight  degree  of  hyperpyrexia  low- 
ers the  resistance  of  the  organism  to  infection. 


CHAPTER  XV. 
THE  BLOOD. 

Introduction. — The  individual  cells  forming  the  most  simple 
types  of  life  are  nourished  by  substances  which  they  obtain 
directly  from  the  water  in  which  the  animal  lives.  In  exchange 
for  this  food,  they  excrete  into  the  water  the  waste  materials  of 
their  metabolism.  As  the  organism  becomes  more  and  more 
complex  this  direct  interchange  of  materials  becomes  impossible, 
and  the  blood  and  lymph  assume  the  task  of  delivering  food  to 
the  tissues  and  of  removing  the  waste  materials.  To  accomplish 
this,  these  fluids  come  into  close  relation  with  the  absorbing,  elimi- 
nating, and  general  tissue  elements  of  the  body,  the  lymph  being 
in  immediate  contact  with  the  cells  and  the  blood  moving  quickly 
from  place  to  place.  Therefore  all  the  elements  found  in  the 
tissues  and  all  the  waste  materials  produced  by  the  body,  are 
present  at  some  time  in  the  blood.  The  blood  may  indeed  be 
compared  to  the  wholesaler  of  commerce,  who  handles  all  the 
materials  for  the  support  of  life,  and  the  lymph  to  the  retailer, 
who  distributes  to  the  tissue  cells  the  materials  which  they  need. 
In  short,  it  may  be  said  that  the  blood  replenishes  the  lymph  for 
the  losses  which  it  incurs  in  supplying  the  tissues. 

Physical  Properties. — Ordinary  mammalian  blood  is  an 
opaqjie,  somewhat  viscid  fluid,  varying  in  color  from  a  bright 
red  in  arterial  blood  to  a  dark  red  in  venous  blood.  Contact  with 
air  changes  venous  blood  to  arterial  blood.  Microscopical  exami- 
nation shows  that  the  blood  is  not  perfectly  homogenous,  but 
consists  of  a  clear  fluid  in  which  cells  called  corpuscles  are  sus- 
pended. 

The  Corpuscles. 

There  are  three  varieties  of  these :  the  red  corpuscles  (to  which 
the  color  of  blood  is  due),  the  wliite  corpuscles  and  the  Ijlood 
platelets. 

140 


THE  BLOOD  CORPUSCLES.  141 

Erythrocytes.— The  red  corpuscles,  or  erytJirocytes,  as  they 
are  called,  are  by  far  the  most  numerous,  there  being  five  mil- 
lion of  them  in  a  cubic  millimeter  of  normal  blood.  Examined 
under  the  microscope,  they  are  seen  in  man  to  be  flattened,  bi- 
concave, non-nucleated  discs;  but  in  the  embryo,  as  well  as  in 
birds  and  reptiles,  they  have  a  nucleus.  Each  corpuscle  consists 
of  an  envelope  and  a  framework  of  protein  and  lipoid  material 
containing  a  substance  known  as  haemoglobin. 

HEMOGLOBIN  is  a  very  complex  body,  belonging  to  the  general 
class  of  compound  proteins  (see  p.  21).  Haemoglogin  has  the 
ability  to  unite  with  large  amounts  of  oxygen,  thus  enabling 
the  blood  to  carry  the  oxygen  gathered  in  the  lungs,  to  the  dis- 
tant tissues.  It  consists  of  a  combination  of  a  simple  protein, 
globin,  and  a  pigment,  heematin.  Hcematin  contains  iron,  which 
is  responsible  for  the  ability  of  oxygen  to  unite  with  the  hasmo- 
globin  molecule.  The  combination  of  haemoglobin  with  oxygen 
is  not  very  stable,  and  can  be  readily  broken  with  the  liberation 
of  oxygen.  It  is  for  this  reason  that  this  molecule  is  adapted  to 
carry  oxygen  to  the  tissues.  The  quantity  of  haemoglobin  held 
by  the  corpuscle  may  vary  and  in  some  diseases,  as  in  chloro- 
anaemia,  for  instance,  it  may  be  greatly  diminished,  so  much  so 
that  the  tissues  may  be  unable  to  obtain  the  proper  amount  of 
oxygen.  The  amount  of  haemoglobin  actually  present  in  a  sample 
of  blood  may  be  estimated  by  the  intensity  of  the  red  color  it 
gives  to  the  blood.  To  estimate  this  intensity  a  drop  of  blood 
is  received  on  blotting  paper,  the  stain  being  then  compared 
either  with  that  produced  by  normal  blood  in  various  dilutions 
on  the  same  paper,  or  with  a  standardized  chart.  From  the  con- 
centration of  normal  blood  whose  stain  most  nearly  matches  that 
of  the  unknown  sample,  we  can  determine  the  percentage  of 
haemoglobin  in  the  latter,  or  we  can  read  this  directly  from  the 
chart. 

Enumeration  of  the  Blood  Corpuscles. — The  number  of  red 
or  white  cells  present  in  a  cubic  millimeter  of  blood  may  be  esti- 
mated by  the  use  of  a  haemocytometer  or  blood-counter.  This 
consists  of  two  mixing  capillary  tubes,  in  one  of  which  the  blood 
is  diluted  one  hundred  times  with  saline  solution,  and  in  the 


142 


HUMAN   PHYSIOLOGY. 


other,  ten  times  with  0.337%  acetic  acid.  The  former  dilution  is 
for  counting  red,  and  the  latter,  for  counting  white  corpuscles. 
A  drop  of  the  diluted  blood  is  then  placed  on  a  special  glass 
slide  which  contains  a  counting  chamber  of  such  a  depth  that 
when  a  cover  slip  is  put  over  a  drop  of  fluid  in  the  chamber,  a 
column  of  fluid  one-tenth  of  a  millimeter  deep  is  obtained  (Fig. 
13).  The  chamber  is  graduated  with  cross  lines,  so  that  each 
square  represents  a  known  fraction  of  a  millimeter.  The  average 
number  of  corpuscles  found  in  a  number  of  squares,  by  actual 
count  with  a  microscope,  is  multiplied  by  the  factors  of  dilution 
employed,  the  product  being  the  number  of  cells  in  a  cubic  milli- 


0.10  Onim. 

o 

C,  Zeiss 

'■'rr—^ 

4qH'^'^^- 

v.:„.^z:-::_i2?' 

Jena. 

Fig.  13. — Thoma-Zeiss  Htemoeytome'ier ,  M,  mouthpiece  of  tube  iG),  by 
which  blood  is  sucked  into  S;  B,  bead  for  mixing;  a,  view  of  slide  from 
above  ;  J),  in  section  ;  c,  squares  in  middle  of  B,  as  seen  under  microscope. 


meter  of  blood.  The  erythrocytes,  which  iii  liealth  number  about 
five  million  in  a  cubic  millimeter,  may  decrease  to  less  than  a 
million  in  disease,  such  as  pernicious  anaemia,  or  after  hsemor- 
rhage.  On  the  other  hand,  they  may  number  six  or  seven  million 
in  people  who  live  at  high  altitudes.  The  oxygen-carrying  power 
of  the  blood  is  proportional  to  the  percentage  of  hemoglobin,  so 
that  by  estimating  this  and  the  number  of  corpuscles,  a  fair  idea 
of  the  condition  of  the  blood  is  obtained. 

The  Origin  of  Erythrocytes. — It  is  interesting  to  inquire 
into  the  source  of  the  blood  cells,  but  although  this  has  been  the 
subject  of  many  researches,  it  is  by  no  means  definitely  settled 


THE  BLOOD  CORPUSCLES.  143 

just  what  the  process  is  or  in  what  part  of  thebody  the  cells  origi 
nate.  Nor  is  it  definitely  known  just  wliere  the  worn  out  cells 
are  dealt  with.  In  the  embrj'^o  certain  cells  are  set  apart  to 
develop  the  vascular  system.  Some  of  these  form  the  blood  ves- 
sels an(J  some  the  red  corpuscles,  but  later  in  foetal  life,  the  latter 
come  from  cells  in  the  spleen,  liver  and  red  bone-marrow.  At 
first  the  red  corpuscles  are  nucleated,  but  towards  the  end  of 
fcetal  life  they  begin  to  lose  their  nuclei,  so  that  at  birth  there 
are  very  few  nucleated  red  corpuscles  remaining  in  the  blood. 
After  birth,  the  red  corpuscles  are  formed  in  the  red  bone-mar- 
row of  the  flat  bones.  In  these  places  special  nucleated  cells  are 
found,  which  are  called  erythroblasts,  and  from  these  the  er^^- 
throcytes  develop.  After  severe  haemorrhage  nucleated  red  cells 
may  apJDcar  in  the  blood  for  a  short  time ;  the  same  is  true  in 
some  forms  of  anaemia  in  which  there  occurs  a  very  rapid  destruc- 
tion accompanied  with  a  very  rapid  formation  of  red  cells. 

Since  the  life  of  an  erythrocyte  is  necessarily  limited,  provision 
must  be  made  for  the  destruction  and  elimination  of  the  sub- 
stances of  which  they  are  composed.  In  the  pigments  of  the  bile 
we  find  the  remains  of  part  of  the  hgemoglobin.  The  bile  is 
secreted  by  the  liver  into  the  intestine  (see  p.  71),  and  in  case 
the  free  outflow  of  bile  is  interfered  with,  the  blood  absorbs  the 
pigment  and  the  individual  becon.es  yellow  or  is  said  to  be  jaun- 
diced. The  bile  pigments  do  not,  however,  contain  all  the  ele- 
ments of  the  haemoglobin,  for  the  iron  is  not  excreted  by  the  bile. 
It  is,  on  the  contrary,  stored  up  bj^  the  liver  to  be  used  again 
in  the  formation  of  fresh  haemoglobin.  Some  have  thought  that 
the  function  of  the  spleen  is  to  destroy  the  red  blood  cells,  the 
waste  products  of  which  are  sent  to  the  liver  through  the  splenic 
vein.  The  evidence  for  this  is  the  presence  of  pigment  and  iron- 
containing  substances  in  the  blood  of  this  vein. 

Iron  is  an  essential  constituent  in  the  haemoglobin  molecule, 
and  it  is  necessary  that  some  be  constantly  supplied  to  the  body 
in  the  food.  But  this  amount  need  not  be  large,  since  the  iron- 
containing  substance  can  be  used  time  and  again  in  the  manu- 
facture of  new  haemoglobin  J  and  once  the  body  has  the  requisite 
amount,  little  more  need  be  added  (see  p.  120).     Indeed,  it  is 


144  HUMAN  PHYSIOLOGY. 

questionable  if  the  inorganic  forms  of  iron  can  be  utilized  by  the 
body,  the  iron  in  our  blood  being  probably  derived  from  a  con- 
jugated protein  known  as  hsematogen,  found  in  small  quantities 
in  the  food. 

The  White  Blood  Cells. — ^In  normal  human  blood  there  are 
about  ten  thousand  cells'  in  a  cubic  millimeter  of  blood,  or  about 
one  to  every  five  hundred  red  cells.  In  many  ways  they  resemble 
the  ujiicellular  amoeba,  for  like  it  they  have  the  power  of  making 
independent  movement  by  extending  tiny  processes  called  pseu- 
dopodia  in  one  direction  and  by  retracting  them  in  another.  By 
virtue  of  this  peculiar  movement  they  are  able  to  flow,  as  it 
were,  between  the  endothelial  cells  of  the  capillaries  and  find 
their  way  into  the  tissue  spaces.  There  are  a  number  of  forms 
of  white  cells  differing  from  each  other  in  size,  in  the  character 
of  their  nucleus,  and  in  the  granules  they  contain.  In  general, 
they  are  classified  in  two  main  groups  on  morphological 
grounds,  viz.,  leucocytes  and  lymphocytes. 

The  leucocytes  are  the  most  numerous  and  compose  about  65 
per  cent  of  the  total  white  cells.  They  are  characterized  by  a 
lobed  nucleus,  the  parts  of  which  are  connected  by  strands  of 
chromatin  material.  To  this  class  belong  several  sub-groups. 
The  most  important  of  these  are  the  cells  known  as  polymorpho- 
nuclear leucocytes.  They  comprise  about  96  per  cent  of  the 
leucocytes.  Others  are  known  as  eosinophiles,  since  they  have_ 
granules  which  have  a  marked  affinity  for  acid  stain^. 

The  Lymphocytes,  the  second  variety,'  are  so-called,  since 
they  are  supposed  to  be  formed  in  the  lymph  glands  of  the  body. 
They  possess  a  single  large  round  nucleus  surrounded  by  a  clear 
layer  of  protoplasm.  There  are  two  sub-groups  in  this  class : 
the  large  mononuclear  lymphocytes,  which  contain  a  rather  abun- 
dant cytoplasm  about  the  nucleus,  and  the  small  mononuclear 
lymphocytes,  in  which  the  amount  of  cytoplasm  is  very  small. 
The  former  comprise  about  4  per  cent,  and  the  latter  about  30 
per  cent,  of  the  white  cells. 

Estimation  of  the  White  Cells. — The  number  of  white  cells 
found  in  the  blood  is  estimated  by  the  same  principle  that  is  em- 
ployed in  the  counting  of  the  red  cells  (see  p.  142).    In  certain 


THE   BLOOD   PLASMA.  145 

diseases  their  number  may  vary  greatly.  The  number  is  also  in- 
creased after  meals.  A  marked  increase  over  normal  is  known 
as  a  leucocytosis. 

The  Function  of  the  Leucocytes. — In  acute  infections,  as 
in  appendicitis,  i:)neumonia,  and  localized  or  general  septic  con- 
ditions in  which  pus  is  formed,  there  is  usually  a  great  increase 
in  the  number  of  the  polymorphonuclear  leucocytes.  In  more 
chronic  infections,  as  in  tuberculosis,  the  lymphocytes  are  found 
in  greater  number.  In  the  parasitic  diseases  of  animal  origin,  as 
tapeworm  and  hookworm,  in  some  skin  diseases,  and  in  scarlet 
fever,  the  eosinophile  leucocytes  are  more  abundant.  In  the 
disease  leucocythsemia  the  lymphocytes  may  be  present  in  such 
great  numbers  that  they  impede  the  movement  of  blood  by  in- 
creasing its  viscosity  or  thickness.  The  above  observations  sug- 
gest that  leucocytes  play  an  important  role  in  the  protection  of 
the  body  from  infective  processes.  This  function  will  be  dis- 
cussed later.  Another  important  function  they  may  have  is  the 
preparation  of  the  peculiar  proteins  which  are  found  in  the 
blood  plasma. 

The  Blood  Platelets. — These  bodies  are  smaller  than  the 
erythrocytes,  and  number  about  300,000  in  a  cubic  millimeter 
of  blood.  When  blood  is  shed  they  disintegrate  very  rapidly, 
and  set  free  a  substance  which  plays  a  part  in  the  coagulation  of 
the  blood.  Little  is  known  concerning  their  chemical  constitution 
or  their  physiological  function. 

The  Blood  Plasma. 

The  blood  plasma  is  a  very  complex  fluid  containing  all  the  va- 
ried substances  associated  with  the  function  of  the  blood.  Water 
composes  90  per  cent  of  the  plasma.  The  plasma  proteins  consti- 
tute the  largest  solid  constituent  (7  per  cent),  and  include  serum 
globulin,  serum  albumin,  and  fibrinogen.  There  are  a  number  of 
bodies  which  contain  nitrogen  which  are  not  proteins.  These 
may  be  grouped  into  two  classes,  the  first,  represented  by  the 
amino  acids  and  other  nitrogenous  bodies  derived  from  the  pro- 
tein of  the  food  and  from  which  the  tissue  cells  are  built,  and  the 
second  group,  represented  by  waste  materials  given  off  by  the 


146  HUMAN   PHYSIOLOGY. 

tissue  cells.  These  include  substances  such  as  urea,  uric  acid, 
creatinin,  and  ammonia.  The  non-nitrogenous  organic  bodies  are 
dextrose,  of  which  0.1  per  cent  is  present  in  normal  plasma,  and  a 
small  quantity  of  fat.  About  1  per  cent  of  inorganic  salts  are 
found,  the  chief  of  which  is  sodium  chloride,  which  constitutes 
60  per  cent  of  the  ash.  Sodium  carbonate  is  found  in  a  little  less 
degree.  Besides*  these  two  we  find  small  amounts  of  potassium, 
sodium  and  calcium  chlorides  and  phosphates.  An  important 
group  of  substances  known  as  hormones  are  excreted  into  the 
plasma  by  some  of  the  glands  of  the  body,  and  affect  the  meta- 
bolism of  the  tissues  in  a  specific  manner.  Another  group  of 
bodies,  the  antitoxins,  complements,  and  opsonins  (see  p.  154), 
are  found  in  the  blood.  These  are  concerned  in  the  protection  of 
the  body  against  infective  organisms. 


CHAPTER  XVI. 

THE  BLOOD  (Cont'd). 

The  Defensive  Mechanisms  of  the  Blood. 

The  Coagulation  of  the  Blood. — ^Whenever  a  blood  vessel  is 
slightly  cut,  the  blood,  which  at  first  comes  very  freely,  soon 
ceases  to  flow  because  of  the  formation  of  a  plug  or  clot  of  blood 
at  the  site  of  the  injury.  The  process  by  which  the  blood  spon- 
taneously forms  the  plug  in  the  injured  vessel  is  known  as  coagu- 
lation, or  clot  formation.  It  protects  the  body  from  fatal  hem- 
orrhage in  case  of  an  ordinary  wound.  A  clot  is  a  semi-solid 
mass,  which  on  microscopical  examination  is  seen  to  consist  of  a 
meshwork  of  fibrils  holding  the  blood  corpuscles  in  their  inter- 
spaces. If  blood  is  collected  in  a  basin  and  whipped  with  some 
twigs  while  it  is  clotting,  the  fibrils  will  collect  on  the  twigs  in 
stringy  masses,  and  the  blood  will  remain  fluid.  The  stringy 
material  is  called  fibrin.  Obviously,  fibrin  cannot  exist  in  the 
blood  stream,  else  the  blood  would  form  a  clot  within  the  blood 
vessels ;  it  is  formed  only  when  occasion  demands,  such  as  an  in- 
jury to  the  blood  vessel.  There  are  a  number  of  experiments 
which  explain  the  process  of  coagulation. 

Thus,  if  blood  is  prevented  from  clotting  by  cooling  it  to  0.° 
Centigrade,  and  is  then  mixed  with  a  saturated  solution  of  salt, 
a  white  precipitate  forms,  which  may  be  filtered  off  and  dissolved 
in  0.1  per  cent  salt  water.  This  solution  may  be  made  to  clot  by 
the  addition  of  a  very  little  blood  from  which  the  fibrin  has  been 
removed.  In  other  words,  we  have  prepared  a  substance  which 
under  proper  conditions  forms  the  fibrin  of  the  clot.  This  sub- 
stance is  called  fibrinogen,  since  it  is  the  precursor  of  fibrin. 

Again,  if  blood  be  treated  with  sodium  oxalate,  it  will  not  clot 
unless  calcium  salts  be  added  in  amount  sufficient  to  precipitate 

147 


148  HUMAN   PHYSIOLOGY, 

completely  all  the  oxalate  and  leave  some  in  excess.  In  other 
words,  the  presence  of  a  soluble  calcium  salt  is  necessary  in  order 
to  have  the  blood  clot.  Defibrinated  blood  will,  however,  cause 
the  clotting  of  pure  fibrinogen  solutions  even  though  all  the  cal- 
cium be  removed  from  both  solutions. 

In  order  to  explain  the  above  facts,  we  must  assume  that  three 
substances  are  present  in  solution  in  the  blood:  fibrinogen,  cal- 
cium salts,  and  another  substance,  which  has  been  called  throm- 
hogen.  Under  the  proper  conditions,  thrombogen  will  combine 
with  calcium  salts  to  form  thrombin,  which  in  turn  unites  with 
fibrinogen  to  fo'rm  fibrin,  which  is  the  substance  forming  the 
framework  of  the  clot. 

The  reason  why  the  blood  does  not  clot  within  the  blood  ves- 
sels is  not  definitely  known.  It  is  probable  that  the  blood  con- 
tains a  substance  which  prevents  the  combination  of  thrombogen 
with  calcium  salts,  and  which  we  call  anti-ihroiiibin.  Whenever 
a  blood  vessel  is  injured,  the  tissues  and  tho  blood  platelets  liber- 
ate a  lipoid  body  called  kephalin,  which  unites  with  the  anti 
thrombin  and  thus  allows  the  formation  of  thrombin  to  take  place 
at  the  site  of  the  wound.  The  whole  process  may  be  graphically 
shown  in  the  following  schema: 

Anti-thrombin  -\-  kephalin  =  inactive  anti-thrombin. 
Thrombogen  -|-  calcium  salts  =  thrombin. 
Thrombin  -)-  fibrinogen  =  fibrin. 
Fibrin  -\-  corpuscles  =  clot. 

Antibodies  in  the  Blood. — The  coagulation  of  the  blood  is 
only  one  of  the  measures  which  are  developed  in  the  blood  for  the 
protection  of  the  animal.  No  less  important  in  this  regard  are 
the  destruction  and  removal  of  toxic  and  injurious  substances 
from  the  body. 

All  the  infectious  diseases  are  caused  by  the  agency  of  micro- 
organisms. The  greater  number  of  these  are  microscopic  plants 
known  as  bacteria  and  fungi ;  some,  however,  are  unicellular  ani- 
mals known  as  protozoa.  It  is  especially  against  the  bacteria  that 
a  method  of  defense  exists  in  the  body ;  the  protozoal  diseases,  on 


THE   DEFENSIVE    MECHANISMS    OF    THE    BLOOD.  149 

the  other  hand — such  as  syphilis,  malaria,  sleeping  sickness  and 
those  caused  by  amoeba  in  the  mouth  and  alimentary  tract — find 
relatively  little  resistance  offered  to  their  growth  in  the  body,  and 
their  destruction  therefore  must  be  for  the  most  part  brought 
about  by  drugs. 

The  Process  of  Inflammation,  which  in  a  general  way  is 
known  by  the  common  symptoms  of  fever,  pain,  swelling  and 
redness,  is  a  sign  of  an  increased  activity  on  the  part  of  the  tis- 
sues in  an  effort  to  destroy  some  foreign  body  which  is  poisonous 
to  the  cells.  Microscopical  examination  of  a  section  of  inflamed 
tissue  will  show  that  the  blood  vessels  are  dilated,  and  that  the 
tissue  spaces  are  infiltrated  with  leucocytes.  It  suggests  that  the 
blood  elements  must  have  a  very  important  part  in  the  process. 
The  study  of  this  function  of  the  body  is  one  of  the  most  inter- 
esting chapters  of  physiological  science,  and  includes  the  ques- 
tions of  immunity  from  disease  and  the  cure  of  infectious  pro- 
cesses. 

Many  pathogenic  organisms  can  be  cultivated  on  artificial 
media  and  the  products  of  their  metabolism  can  then  be  studied. 
It  has  been  found  that  they  may  be  divided  into  two  groups :  the 
one  group  producing  the  soluble  poisons,  or  true  toxins,  which 
are  excreted  from  the  cell;  and  the  other  producing  toxic  sub- 
stances, the  endo-toxins,  which  are  not  excreted  from  the  cell. 
We  will  first  take  up  the  manner  in  which  the  body  deals  with 
the  toxins. 

Toxins. — If  a  culture  of  diphtheria  or  tetanus  bacilli  be  fil- 
tered through  a  porcelain  filter,  the  bodies  of  the  bacilli  are  re- 
moved and  the  filtrate  contains  the  soluble  toxic  principles  which 
the  bacilli  have  produced  and  excreted  into  the  nutrient  fluid. 
Injections  of  a  small  amount  of  this  filtrate  into  an  animal  will 
produce  the  same  symptoms  as  are  produced  when  a  pure  culture 
of  the  bacilli  is  injected.  Each  bacillus  produces  a  specific  kind 
of  toxin.  Diphtheria  toxin  acts  primarily  on  the  vascular  sys- 
tem; tetanus  toxin,  on  tlie  central  nervous  system.  The  chemical 
nature  of  the  toxin  molecule  is  unknown,  since  it  has  been  impos- 
sible to  separate  it  in  pure  form.  It  is  probably  closely  related 
to  the  protein  molecule,  and  on  the  other  hand  resembles  the 


150  HUMAN   PHYSIOLOGY. 

ferments  in  many  of  its  actions  (see  p.  34).  A  peculiarity  in 
the  action  of  the  toxins  is  that  a  relatively  long  period  elapses 
between  the  injection  of  the  toxin  and  the  reaction  of  the  body, 
whereas  in  the  case  of  the  alkaloids  or  vegetable  poisons,  the  re- 
action appears  very  quickly. 

Antitoxin. — ^In  spite  of  the  very  poisonous  character  of  the 
toxin  molecule,  the  body  is  provided  with  a  means  of  defense 
against  it,  and  is  able  to  make  itself  still  further  immune  to  the 
action  of  the  toxin.  Thus,  if  somewhat  less  than  the  fatal  dose 
of  diphtheria  or  tetanus  toxin  be  injected  into  the  body,  certain 
symptoms  will  follow,  and  the  animal  will  react  to  the  toxin  in 
such  a  way  that  a  subsequent  injection  can  be  made  larger  with- 
out proving  fatal.  If  successively  increasing  doses  are  given,  the 
animal  after  some  weeks  will  be  able  to  withstand  very  large 
doses  of  the  toxin.  In  other  words,  the  body  develops  an  im- 
munity towards  the  toxic  agent;  it  produces  an  antibody  which 
neutralizes  the  poison  of  the  toxin.  To  this  body  we  give  the 
name  of  antitoxin.  Since  these  antibodies  are  found  in  solution 
in  the  blood,  it  is  possible  to  withdraw  the  blood  from  such  an 
immune  animal,  and  inject  it  into  a  non-immune  animal,  thus 
rendering  the  latter  immune  to  the  toxin.  It  is  this  principle 
that  is  used  in  the  preparation  of  diphtheria  and  tetanus  anti- 
toxins. The  exact  nature  of  the  combination  of  the  toxin  and  the 
antitoxin  cannot  be  learned  from  chemical  studies,  but  Ehrlieh 
has  given  to  the  phenomenon  a  biological  explanation  based  on 
the  various  known  reactions  of  the  bodies. 

Ehrlieh 's  Side  Chain  Theory  of  Immunity. — Briefly  summar- 
ized Ehrlieh 's  theory  is  as  follows:  Each  toxin  molecule  is 
made  up  of  a  central  nucleus  of  chemical  radicles  similar  to  those 
found  in  organic  compounds.  To  the  main  body  of  this  mole- 
cule are  attached  at  least  two  other  radicles,  or  side  chains.  One 
of  these  has  a  great  affinity  for  certain  chemical  constituents  of 
the  tissues  of  susceptible  animals,  and  unites  the  toxin  molecule 
to  the  tissue  cell.  This  chain  is  known  as  the  haptophore  group. 
The  other  side  chain,  the  toxophore  group,  exerts  the  injurious 
effect  upon  the  tissue  after  the  haptophore  group  has  joined  the 
toxin  to  the  cell.     For  example,  tetanus  toxin  owes  its  effect  to 


THE    DEFENSIVE    MECHANISMS    OF    THE   BLOOD.  151 

the  fact  that  nervous  tissue  contains  a  chemical  substance  which 
unites  readily  with  the  haptophore  group  of  the  tetanus  toxin, 
and  also  substances  that  are  readily  attacked  by  the  toxophore 
group  of  the  toxin.  The  antitoxins  are  supposed  to  act  by  com- 
bining with  the  haptophore  group,  thus  preventing  the  toxin 
from  uniting  with  the  cell. 

According  to  this  theory  the  formation  of  antitoxins  may  be 
accounted  for  as  follows :  When  a  receptor,  as  we  may  term  the 
portion  of  the  cell  which  unites  with  the  haptophore  groups,  is 
united  to  the  toxin,  the  cell  endeavors  to  adapt  itself  to  the  loss  of 
this  radicle  by  the  production  of  another  similar  one.  Since  the 
general  rule  of  nature  is  to  respond  to  an  action  with  an  over- 
reaction,  many  more  receptors  are  made  than  are  actually  needed 
to  unite  with  the  haptophore  groups  of  the  toxin  present.  The  re- 
ceptors produced  in  such  great  number  break  away  from  the 
parent  cell.  These  accordingly  are  stored  up  in  the  blood,  and 
whenever  any  of  the  particular  toxin  for  which  they  are  adapted 
is  present  in  the  circulation,  they  unite  with  it  and  thus  prevent 
the  toxin  from  uniting  with  the  tissue  cells.  A  body  which  pos- 
sesses a  store  of  such  antibodies  is  said  therefore  to  be 
immune. 

Toxins  are  not  the  only  substances  which  will  produce  specific 
antibodies.  This  property  is  a  general  characteristic  of  proteins. 
Any  substance  producing  an  antibody  is  known  as  an  antigen. 
For  example,  if  human  blood  be  injected  into  a  rabbit,  and  after 
several  da3^s  some  of  the  rabbit's  blood  serum  is  mixed  with  hu- 
man blood  serum,  a  precipitate  will  form,  whereas  the  blood  of 
a  normal  rabbit  will  produce  no  such  precipitate.  The  first  in- 
jection of  human  blood  serves  to  stimulate  the  rabbit  cells  to 
form  some  substance  which  precipitates  any  human  blood  sub- 
sequently added.  The  reaction  is  specific,  for  the  blood  of  any 
other  species  of  animal  will  not  be  precipitated  by  blood  from  a 
rabbit  sensitized  with  human  blood,  and  the  reaction  offers  a  very 
accurate  method  of  differentiating  between  human  blood  and 
other  blood  in  medico-legal  cases.  The  body  thus  formed  is  known 
as  a  prccipilin. 

Anaphylaxis. — Again,  if  a  rabbit  be  injected  with  some  liu- 


152  HUMAN   PHYSIOLOGY. 

man  serum  two  or '  three  weeks  after  a  previous  injection,  the 
animal  will  go  into  a  very  profound  state  of  shock.  The  blood 
pressure  will  be  lowered,  the  heart's  action  weakened,  and  breath- 
ing interfered  with.  This  condition  is  known  as  anaphylactic 
shock.  The  reaction  is  a  general  one  for  proteins  and  is  specific 
for  each  protein  used.  The  phenomenon  is  explained  by  assum- 
ing that  the  first  injection,  while  producing  the  bodies  which  we 
referred  to  above  as  precipitins,  also  produces  an  excess  of  a  fer- 
ment which  is  able  to  break  down  the  foreign  protein  very  quick- 
ly when  the  second  injection  takes  places.  The  products  of  the 
broken  protein  molecule,  as  they  are  produced  in  the  blood,  are 
poisonous  to  the  body  and  produce  the  phenomenon  above  de- 
scribed. 

Phagocytosis. — By  far  the  greater  number  of  pathogenic  or- 
ganisms do  not  excrete  a  poisonous  toxin  into  the  surrounding 
medium,  but  they  cause  disease  by  directly  attacking  the  tissues. 
The  diphtheria  bacillus  does  not  enter  the  body,  but  only  ex- 
cretes a  soluble  toxin  which  the  body  absorbs.  When  a  disease 
involves  the  infection  of  the  tissues  themselves  by  a  micro-or- 
ganism, other  types  of  defense  than  those  described  above  are 
used.  This  defense  depends  on  the  fact  that  some  of  -the  leu- 
cocytes of  the  blood  and  lymph  have  the  ability  to  ingest  and  de- 
stroy foreign  bodies  which  are  present  in  the  blood  and  tissues, 
in  much  the  same  way  as  the  amoeba  takes  its  food.  This  func- 
tion of  the  leucocytes  to  destroy  foreign  bodies  is  known  as  pha- 
gocytosis. In  the  changes  which  accompany  the  metamorphosis 
of  certain  forms  of  larva,  the  leucocytes  are  the  agents  which  re- 
move those  parts  of  the  body  which  are  no  longer  of  service  to 
the  animal.  Likewise  the  leucocytes  of  the  blood  can  be  shown 
to  ingest  pathogenic  bacteria  and  to  destroy  them.  The  exact 
function  of  the  different  varieties  of  white  cells  in  the  blood  is 
not  definitely  known.  In  active  inflammatory  processes  the  poly- 
morphonuclear leucocytes  are  by  far  the  most  numerous.  On 
the  other  hand,  in  cases  of  chronic  infection,  as  in  tuberculosis 
the  number  of  lymphocytes  is  increased.  Some  of  the  forms  of 
white  cells  do  not  take  an  active  part  in  the  ingestion  of  bacteria, 
and  therefore  cannot  directly  destroy  them.    Yet,  in  the  defense 


THE   DEFENSIVE    MECHANISMS   OF    THE   BLOOD.  153 

of  the  organism,  they  take  a  part  Avhich  is  no  less  important  than 
that  of  the  phagocyte. 

In  very  simple  forms  of  life  the  cells  of  the  alimentary  tract 
both  ingest  and  digest  the  food  material.  In  higher  forms  the 
cells  of  the  alimentary  tract  secrete  the  fluids  which  digest  the 
food.  In  the  one  case  the  digestion  is  intra-cellular,  and  in  the 
latter,  extra-cellular.  In  the  same  way  we  find  the  blood  leu- 
cocytes able  both  to  destroy  and  to  digest  substances  by  intra- 
cellular action,  and  also  sharing  with  other  cells  of  the  body  the 
power  to  secrete  substances  into  the  blood  plasma  which  have  the 
power  of  destroying  the  organisms  or  toxic  material. 

Opsonins. — Normal  blood  serum  has  a  very  strong  destruc- 
tive influence  on  most  species  of  bacteria,  whether  they  are  patho- 
genic or  not.  This  ability  is  not  possessed  to  the  same  extent  by 
the  blood  plasma.  The  difference  is  explained  by  the  fact  that 
in  the  process  of  coagulation  the  white  blood  cells  are  broken 
down  and  liberate  their  bactericidal  bodies.  Extracts  made  of 
leucocytes  have  this  same  effect,  but  the  reaction  is  much  more 
rapid  in  the  presence  of  blood  plasma  or  serum.  The  co-oper- 
ation on  the  part  of  the  plasma  or  serum  is  explained  by  the 
presence  of  some  substance  in  solution  which  enables  the  leu- 
cocytes to  attack  the  bacteria  more  readily. 

That  some  such  substances  also  aid  in  the  phagocytic  action  of 
the  leucocytes  is  indicated  by  the  fact  that  the  white  cells  ingest 
bacteria  much  more  quickly  in  blood  serum  than  in  normal  saline 
solution.  These  substances  are  known  as  opsonins,  and  are  char- 
acteristic for  each  individual  organism  which  stimulates  their 
production.  At  the  beginning  of  an  infective  process,  in  which 
the  phagocytosis  is  very  active,  each  leucocyte  may  be  able  to  at- 
tack only  one  or  two  bacteria ;  later  in  the  disease,  however,  when 
the  opsonic  power  has  been  increased  for  the  infective  agent,  the 
leucocytes  may  be  able  to  ingest  a  much  larger  number  without 
injury  to  themselves.  The  opsonic  index  is  a  figure  expressing 
the  ratio  of  the  number  of  pathogenic  organisms  of  a  certain 
kind  that  a  normal  leucocyte  can  ingest  in  serum,  to  that  which 
the  same  leucocyte  can  ingest  in  the  presence  of  the  serum  of  a 


154  HUMAN   PHYSIOLOGY. 

patient  who  is  suffering  from  the  infective  agent.  A  high  op- 
sonic index  therefore  indicates  a  relative  imiminity  or  high  resist- 
ance to  the  disease  in  question.  The  bactericidal  power  of  the 
leucocytes  for  many  bacteria  can  be  greatly  increased  by  the  in- 
jection of  dead  bacteria  into  the  body.  This  fact  is  made  use  of 
in  the  manufacture  of  bacterial  vaccines,  which  consist  of  sus- 
pensions of  dead  bacteria  in  a  saline  solution. 


CHAPTER  XVII. 
THE  LYMPH. 

The  blood  circulates  in  closed  tubules,  so  that  the  uourisliment 
which  is  supplied  the  tissues  and  the  effete  products  which  re- 
sult from  their  activity  must  pass  through  the  walls  of  the  ves- 
sels. The  fluid  which  is  transuded  from  the  capillaries  and  which 
.surrounds  the  cells  of  the  tissues  is  known  as  the  lymph,  and 
serves  as  the  medium  of  exchange  between  the  cells  and  the  blood 
plasma.  It  is  the  middleman  of  exchange  between  the  blood  and 
the  tissues.  Lymph  is  a  slightly  yellow  transparent  fluid,  closely 
resembling  the  blood  plasma  from  which  it  is  derived.  To  aid  in 
returning  the  lymph  to  the  blood,  there  is  provided  a  special 
sj-stem  of  vessels  called  the  lymphatics,  which  are  very  thin- 
walled  capillary  tubules  lined  with  endothelial  cells.  These  tu- 
bules lead  to  larger  ones  which,  after  passing  through  a  lymph 
gland  along  their  course,  finally  empty  into  a  large  vein-like  ves- 
sel, the  thoracic  duct,  lying  alongside  of  the  oesophagus  in  the 
thorax,  and  emptying  into  the  left  subclavian  vein.  A  smaller 
lymphatic  vessel,  the  right  thoracic  duct,  empties  into  the  right 
subclavian  vein. 

The  lymph  obtained  from  the  thoracic  duct  by  means  of  a  fine 
tube  inserted  into  the  vessel  varies  somewhat  in  nature.  After  a 
meal  the  fluid  is  like  milk,  because  of  the  presence  of  droplets  of 
fat  which  have  been  absorbed  from  the  intestines.  The  lymphatics 
of  the  viscera  appear  as  white  lines  in  the  mesentery  and  on  this 
account  are  called  lacteals.  The  lymph  which  is  collected  during 
a  fast  is  very  much  like  the  blood  plasma.  Its  specific  gravity  is 
less  than  that  of  blood,  since  it  contains  less  protein  material,  but 
on  the  other  hand  its  salt  content  is  the  same  and  it  clots  in  much 
the  same  manner  as  blood.  On  microscopic  examination  there 
are  found  many  colorless  corpuscles,  identical  with  those  present 
in  blood.    fSome  of  these  corpuscles  are  formed  within  the  lymph 

155 


156  HUMAN   PHYSIOLOGY. 

glands  through  which  the  lymph  vessels  pass  on  their  way  to  the 
subclavian  vein. 

Lymph  Formation. — Many  physiologists  have  attempted  to 
discover  the  precise  mechanism  by  which  the  plasma  passes 
through  the  capillary  walls  into  the  lymph  spaces,  but  the  com- 
plete knowledge  of  the  process  is  not  yet  at  hand.  The  relatively 
high  blood  pressure  within  the  capillaries  provides  filtration 
pressure  by  which  a  fluid  might  be  filtered  through  the  capillar}^ 
walls,  and  there  is  no  doubt  that  such  a  process  does  occur,  as,  Eor 
example,  after  the  capillary  pressure  has  been  increased  by  con- 
striction of  the  veins  by  a  bandage,  etc.  Filtration,  however, 
cannot  explain  all  the  known  phenomena  of  lymph  formation. 
Osmosis  (p.  27)  also  plays  a  part  as  follows:  The  tissues  use  up 
the  nutritional  elements  brought  to  them  by  the  lymph.  The 
diffusion  pressure  of  the  substances  in  the  lymph  is  now  reduced 
so  that  it  becomes  less  than  that  present  in  the  blood.  Therefore 
substances  within  the  blood  must  pass  out  through  the  capillary 
walls  into  the  lymph,  thus  keeping  the  concentration  of  the  fluid 
more  or  less  constant.  The  waste  products  of  the  tissue  pass  into 
the  lymph  and,  by  increasing  the  molecular  concentration  of  the 
lymph,  draw  water  from  the  blood.  Again,  the  breaking  down  of 
the  large  protein  molecules  into  smaller  ones,  in  the  processes  of 
tissue  metabolism,  will  cause  the  molecular  concentration  of  the 
tissues  to  rise,  increasing  the  osmotic  pressure.  This  causes  water 
to  be  abstracted  from  the  lymph,  which  in  turn  draws  on  the 
blood  for  water. 

Lymphagogues. — There  are  certain  substances  which  affect 
the  rate  of  lymph  formation  in  a  very  peculiar  way.  These  are 
called  lymphagogues,  and  include  extracts  from  many  shell  fish, 
leech  extract,  peptones,  etc.  When  such  substances  are  injected 
into  the  blood  of  an  animal,  there  follows  a  great  increase  in  the 
rate  of  lymph  formation  and  lymph  flow.  Indeed  some  people 
are  very  susceptible  to  this  action,  and  eating  shell  fish,  oysters, 
and  some  fruits  will  cause  their  tissues  to  become  swollen  be- 
cause of  an  increased  lymph  formation.  How  these  substances 
can  effect  the  change  by  altering  the  physico-chemical  constitu- 
tion of  the  blood  plasma  is  not  clear.    Some  investigators  believe 


THE  LYMPH.  157 

that  they  have  a  stimiilatmg  action  on  the  endothelial  cells  lining 
the  capillaries  and  thus  produce  an  actual  secretion  of  lymph. 
It  is  more  probable,  however,  that  they  poison  these  cells  in  a 
way  which  increases  their  permeability  and  thus  permits  a  freer 
filtration  of  lymph  from  the  blood  plasma.  There  are  other 
facts  nevertheless  which  support  the  theory  of  an  actual  secreting 
mechanism  within  the  cells  of  the  capillary  walls,  but  they  are 
too  technical  to  consider  here.  They  suggest  that  although  the 
physico-chemical  laws  of  diffusion,  osmosis,  filtration,  etc.,  play 
the  most  important  role  in  lymph  formation,  the  cells  of  the 
capillary  walls  may  themselves  have  an  active  part  in  the  pro- 
cess. 

Lymph  Reabsorption. — Within  the  tissue  spaces,  and  within 
the  cells  of  the  tissues,  changes  are  continually  taking  place  which 
alter  the  character  of  the  lymph.  Oxygen  and  food  substances 
are  removed  from  the  lymph  by  the  tissue  cells,  and  waste  sub- 
stances, the  result  of  the  tissue  metabolism,  are  added  to  it.  In 
the  case  of  oxygen  and  carbon  dioxide,  the  exchange  is  so  reg-, 
ulated  as  to  keep  constant  the  supply  of  these  bodies  in  the 
lymph.  The  loss  of  any  substance  is  quickly  compensated  for  by 
the  addition  of  new  material  from  the  blood.  The  solid  waste 
matter  excreted  by  the  cell  can  also  find  its  way  directly  from 
the  cell  through  the  lymph  and  into  the  blood  plasma.  It  is 
probable  that  during  periods  of  rest  or  of  slight  activity  the 
lymphatics  are  of  little  importance  in  the  exchange  of  the  lymph. 
However,  when  the  exudation  of  lymph  becomes  increased,  as 
during  exercise  or  following  the  use  of  some  lymphagogue,  or 
when  there  are  substances  in  the  lymph  which  the  capillaries 
cannot  absorb  into  the  blood,  the  lymphatics  become  very  im- 
portant in  helping  to  remove  the  excess  of  lymph  formed. 

The  Movement  of  the  Lymph. — The  mechanism  by  which  the 
lympli  of  the  tissues  is  collected  by  the  capillaries  of  the  lymph- 
atic system  is  not  understood  any  better  than  the  mechanism  of 
lymph  formation,  but  no  doubt  the  same  laws  apply  to  both  pro- 
cesses. The  movement  of  the  lymph  along  the  lymphatic  vessels 
is  po.ssible  because  of  tlie  presence  of  valves  along  the  course  of 
the  vessels. 


158  HUMAN   PHYSIOLOGY. 

The  process  of  lymph  absorption  is  rather  slow  except  when  it 
is  aided  by  the  massage  produced  by  the  movements  of  the  sur- 
rounding parts.  The  rapid  action  of  poisons,  or  drugs  intro- 
duced by  a  hypodermic  syringe,  is  due  to  their  absorption  from 
the  intra-cellular  or  lymph  spaces  directly  into  the  blood.  Col- 
ored solutions  as  india  ink  are  absorbed  by  the  lymphatics,  and 
by  using  a  substance  like  this  it  is  possible  to  trace  the  lymphat- 
ics of  a  portion  of  the  body.  Micro-organisms,  such  as  the  strep- 
tococcus, which  causes  one  of  the  familiar  forms  of  what  is  known 
as  blood  poisoning,  are  taken  up  by  the  lymphatics,  and  it  is 
easy  to  trace  the  channels  traversed  by  the  organism  by  the  in- 
flamed lymphatic  walls  which  appear  as  red  lines  under  the  skin. 
Since  all  these  vessels  pass  through  a  lymphatic  gland  on  their 
way  to  the  subclavian  vein,  these  glands  are  often  very  much 
swollen,  and  may  even  be  destroyed  as  the  result  of  the  infection. 
It  is  probable  that  one  of  the  functions  of  the  lymph  gland  is  to 
catch  and  render  non-toxic,  poisons  which  are  being  carried  into 
the  circulation  by  way  of  the  lymphatics.  One  of  the  most 
dreaded  diseases,  carcinoma,  is  carried  by  the  lymphatic  system 
to  other  parts  of  the  body.  For  this  reason  we  most  often  see  the 
metastatic  growths  of  cancer  in  the  region  of  the  lymph  glands 
which  have  caught  the  straying  cancer  cell  and  have  been  infect- 
ed by  it. 

The  increased  exudation  of  lymph  in  the  tissues  which  occurs 
in  inflammatory  conditions  is  no  doubt  of  great  advantage  to  the 
tissues,  since,  by  this  means,  a  greater  supply  of  nourishment  is 
provided  for  the  repair  of  the  damaged  cells,  and  the  defensive 
substances  (antibodies,  etc.)  are  brought  into  play. 


Fig.  14. — Diagram  of  Circulation.  The  blood  circulates  as  follows:  V.C. 
(vense  cavse),  B.A.  (right  auricle),  R.V.  (right  ventricle),  P. A.  (pulmonary 
artery),  P.V.  (pulmonary  vein,  red),  L.A.  (left  auricle),  L.V.  (left  ventricle), 
A. A.  and  D.A.  (ascending  and  descending  aorta),  H.V.  and  B.  (capilaries  of 
head,  viscera  and  body  generally),  P.V.  (portal  vein,  blue),  Li.  (liver).  The 
small  black  vessels   are   the   azyfios  veins. 


CHAPTER  XVIII. 
THE  CIRCULATORY  SYSTEM. 

Introduction. — The  circulatory  system  provides  for  the  trans- 
portation of  blood  through  the  tissues,  thus  enabling  each  indi- 
vidual cell  to  obtain  nourishment  and  to  rid  itself  of  the  waste 
products  of  its  activity.  The  system  includes  the  heart,  the 
blood  vessels,  and  the  lymphatics. 

From  a  mechanical  standpoint,  we  may  say  that  the  heart  con- 
sists of  a  pair  of  pumps ;  each  pump  consisting  of  two  parts,  an 
upper  chamber,  the  auricle,  and  the  lower  one,  the  ventricle. 
Thin,  membranous  valves,  called  auriculo-ventricular,  separate 
the  upper  and  lower  chambers  and  prevent  the  blood  from  flow- 
ing back  into  the  auricle  when  the  ventricle  contracts.  Connect- 
ed with  the  ventricles  are  the  arteries,  which  conduct  the  blood 
away  from  the  heart,  to  which  it  is  returned  by  the  great  veins 
leading  into  the  auricles.  At  the  point  where  the  arteries  emerge 
from  the  heart  are  cup-shaped  valves,  called  semilunar,  which 
prevent  the  passage  of  blood  from  the  arteries  into  the  ventricles 
while  the  latter  are  relaxing. 

As  will  be  seen  from  the  accompanying  diagram  (Fig.  14)  the 
blood  pumped  from  the  two  sides  of  the  heart  circulates  through 
two  distinct  and  separate  systems  of  blood  vessels.  From  the 
right  ventricle  the  blood  goes  through  the  pulmonary  artery  to 
the  lungs  and  is  returned  to  the  left  auricle  by  the  pulmonary 
veins,  then  to  the  left  ventricle,  whence  it  is  sent  over  the  body 
through  the  aorta  and  its  branches,  to  the  capillaries  imbedded 
in  the  tissues.  From  these  it  is  returned  through  the  veins  to  the 
venae  cava?,  which  discharge  it  into  the  right  auricle.  We  may 
say,  therefore,  that  the  circulatory  system  consists  of  two  circles 
of  tubing  interposed  in  which  are  two  force  pumps,  the  valves 
of  which  are  so  disposed  as  to  allow  the  blood  to  flow  in  one  direc- 
tion only. 

159 


160 


HUMAN   PHYSIOLOGY. 


I.     The  Heart. 

Anatomical  Considerations. — The  heart  is  suspended  at  its 
base  by  the  large  arteries,  and  lies  practically  free  in  a  sac  of 
tough  fibrous  tissue  called  the  pericardium.  On  each  side  are  the 
lungs,  with  the  diaphragm  below,  the  chest  wall  in  front,  and  thy 
oeKSophagus  behind  (Fig.  15).  The  surface  of  the  heart  and  the 
interior  of  the  pericardial  sac  are  bathed  with  a  serous  fluid,  the 
pericardial  fluid.  The  muscular  fibers  forming  the  walls  of  the 
four  chambers  of  the  heart  are  arranged  so  that  their  contrac- 


Fig.  15. — The  position  of  tlie  heart  in  the  thorax.    (T.  Wingate  Todd.) 


tion  diminishes, the  size  of  the  cavities  and  empties  the  heart  of 
blood. 

From  the  study  of  the  embryonic  heart,  and  from  comparative 
studies  in  the  lower  animals  (Fig.  16),  we  know  that  the  heart 
has  developed  from  a  single  tube,  the  division  of  the  auricles  and 
the  ventricles  being  a  rather  late  stage  in  the  development  of  the 
mammalian  heart.  The  fact  that  the  two  auricles  beat  synchro- 
nously, followed  by  the  contraction  of  the  two  ventricles,  is  signi- 
ficant of  the  development  of  the  auricles  from  the  proximal,  and 


ANATOMY   OP  THE  HEART. 


161 


of  the  ventricles  from  the  distal  end  of  the  primitive  cardiac 
tube. 

The  fibers  of  the  auricles  run  transversely,  beginning  and  end- 
ing in  the  fibrous  tissue  which  separates  the  auricles  from  the 
ventricles.  The  musculature  of  the  ventricles  is  somewhat  hard- 
er to  trace.  There  are  layers  that  run  transversely  around  the 
ventricles,  and  also  layers  which  describe  more  or  less  of  a  spiral 
course  from  the  base  of  the  ventricles  to  the  apex  and  then  are 
reflected  back  in  transverse  layers,  until  they  finally  end  in  the 
papillary  muscles,  which  are  connected  with  fibrinous  threads. 


Fig.  16. — A  generalized  view  of  the  vertebrate  lieart  (Keith)  showing:  a, 
the  sinus  venosus ;  b.c,  the  auricle ;  S3,  the  auriculo-ventricular  orifice  and 
valves;  d,  the  ventricle;  e,  the  beginning  of  the  aorta  with  the  semilunar 
valves  at  5.  The  valves  between  e  and  /  do  not  exist  in  the  heart  of  man. 
(From  Howell's  Physiology.) 

the  chordae  tendineae,  to  the  edge   of  the  auriculo-ventricula*r 
valves. 

When  the  ventricles  contract,  this  arrangement  of  muscular 
fibers  causes  the  apex  and  the  base  of  the  heart  to  approach  one 
another,  and  the  transverse  section  is  changed  from  an  ellipse  to 
a  circle.  The  base  of  the  heart,  hung  as  it  is  to  the  large  vessels 
in  the  thorax,  ajjpears  to  be  fixed,  and  one  would  expect  that  the 
apex  is  the  part  which  moves  up  and  down.    This  is  not  the  case. 


162 


HUMAN  PHYSIOLOGY. 


however,  as  is  shown  by  experiment,  and  is  explained  by  the  fact 
that  the  blood,  when  it  is  forced  from  the  ventricle  during  the 
cardiac  contraction,  exerts  its  force  on  the  apex  as  well  as  on  the 
blood  in  the  arteries.  This  serves  to  fix  the  apex  in  the  vertical 
position  and  to  bring  the  base  of  the  ventricles  downwards 
during  their  contraction.  In  some  individuals  there  is  a 
visible  pulsation  at  about  the  level  of  the  fifth  rib  on  the  left  side. 
This  is  called  the  apex  heat,  and  is  caused  by  the  rotation  of  the 
apex  in  the  transverse  diameter  and  by  the  sudden  change  of  the 
ventricle  from  a  soft  flabby  condition  into  a  firm  one. 


Fig.  17. — Diagram  of  Valves  of  the  Heart.  The  valves  are  supposed  to  be 
viewed  from  above,  the  auricles  having  been  partially  removed.  A,  aorta 
with  semilunar  valve  ;  B,  pulmonary  artery  and  valve  ;  C,  tricuspid,  and  D, 
mitral  valve  ;  E,  right,  and  F,  left  coronary  artery ;  G,  wall  of  right,  and  H, 
of  left  auricle;  I,  wall  of  right,  and  J,  of  left  ventricle.  (From  Stewart's 
Physiology. ) 


The  walls  of  the  auricles  are  relatively  thin,  as  they  are  not 
required  to  do  heavy  work.  The  ventricular  muscles,  on  the 
other  hand,  are  well  developed,  that  of  the  left  ventricle  being 
very  strong  and  adapted  to  the  heavy  work  it  must  perform. 

The  valves  guarding  the  opening  between  the  auricles  and 
ventricles  are  composed  of  thin  membranes  of  fibrous  tissue,  cov- 
ered with  endothelial  cells  similar  to  the  lining  of  the  heart  and 
the  blood  vessels  (Fig.  17).  In  acute  rheumatism  and  tonsil- 
litis, the  endothelial  covering  of  the  interior  of  the  heart  and  of 
the  valves  is  often  inflamed,  and  permanent  changes  may  take 


THE  HEART  BEAT  163 

place  which  injure  the  valves  and  produce  what  is  known  as  val- 
vular disease  of  the  heart.  The  chordfe  tendineae  connect  the 
free  margins  of  the  valves  with  the  papillary  muscles,  which  arise 
from  the  musculature  of  the  ventricle  like  little  knobs  of  tissue. 
This  arrangement  prevents  the  valves  from  being  everted  into 
the  auricle  during  the  contraction  of  the  ventricle.  The  valves 
on  the  left  side  consist  of  two  flaps  and  are  called  the  mitral 
valves;  those  on  the  right  side  have  three  flaps  and  hence  are 
called  tricuspid  valves.  The  valves  guarding  the  arterial  orifices 
consist  of  three  cup-shaped  membranes  and  are  known  as  the 
semilunar  valves,  because  of  their  crescent-shape  when  they  are 
closed.  Whenever  the  pressure  in  the  arteries  is  greater  than 
that  in  the  ventricles,  these  valves  are  tightly  closed,  and  prevent 
any  blood  entering  the  ventricle  from  the  arteries. 

The  Physiolog'ic  Properties  of  Heart  Muscle. 

The  Character  of  Cardiac  Contraction. — The  contraction  of 
our  voluntary  muscles  is  not  due  to  a  single  stimulus  sent  from 
the  brain  through  the  nerves,  but  rather  to  a  series  of  such  stim- 
uli, which  produce  a  more  or  less  continued  or  tonic  contraction 
of  the  muscle.  If  this  were  not  the  ease,  our  movements  would 
be  very  quick  and  jerky,  similar  to  those  made  by  a  person  suf- 
fering with  St.  Vitus  dance.  In  the  ease  of  the  heart  muscle, 
however,  each  beat  consists  of  a  single  complete  muscular  con- 
traction, and  it  is  impossible  to  produce  a  tonic  or  continued  con- 
traction in  the  heart  such  as  can  be  produced  in  voluntary  mus- 
cle by  rapid  successive  stimuli.  Another  peculiarity  of  heart 
muscle  is  that  each  time  it  contracts  it  does  so  with  all  the  force 
that  it  has  at  the  moment.  Skeletal  muscle  contracts  with  great- 
er or  less  intensity  according  to  the  strength  of  the  stimulus  it 
receives. 

Heart  muscle,  and  in  a  lesser  degree  some  other  muscles,  such 
as  those  of  the  intestinal  tract  and  spleen,  have  the  power  of 
making  automatic  rhythmic  contractions  which  follow  each  other 
in  a  definite  sequence.  This  phenomenon  in  the  case  of  cardiac 
muscle  is  not  dependent  on  the  influence  of  the  nerves,  as  can  be 
shown  by  the  fact  that  the  heart  removed  from  the  body  will  con- 


164  HUMAN   PHYSIOLOGY. 

tinue  to  beat  for  some  time  if  it  is  properly  nourished  by  perfus- 
ing blood  tlirough  it  under  pressure.  The  cause  of  this  prop- 
erty of  automaticity  is  still  unsettled,  and  there  have  been  some 
very  interesting  discussions  and  arguments  among  physiologists 
concerning  it.  Some  believe  that  the  heart  muscle  has  this  prop- 
erty inherent  in  itself,  and  that  it  originates  the  impulse  which 
causes  the  contraction  of  the  heart ;  while  others  think  that  there 
are  present  in  the  heart-muscle  cells  of  a  nervous  character  whose 
special  function  it  is  to  originate  the  beat.  Experimental  facts 
can  be  found  in  support  of  either  theory,  but  the  question  is  still 
in  dispute.  Heart  muscle  differs  from  other  muscle  in  that  each 
fiber  consists  of  a  single  cell  containing  striated  protoplasm.  It 
may  quite  well  be  that  this  kind  of  muscle  possesses  some  char- 
acteristics usually  ascribed  to  nervous  tissue,  and  that  it  does 
originate  the  stimuli  which  produce  automatic  movements. 

The  Sequence  of  the  Heart  Beat. — Inspection  of  the  beating 
heart  of  a  recently  killed  turtle  or  frog  shows  that  the  heart  beat 
begins  by  a  contraction  in  the  large  veins  where  they  join  the 
auricles.  From  these  vessels  the  beat  spreads,  as  it  were,  to  the 
auricles  and  then  to  the  ventricles,  beginning  at  the  base  and 
ending  at  the  apex.  It  is  possible  to  stop  the  contraction  of  the 
ventricles  by  drawing  a  thread  tightly  around  the  heart  between 
the  auricles  and  the  ventricles.  The  auricles  will  continue  to 
beat  as  before,  and  the  ventricles  can  be  made  to  beat  rhythmical- 
ly again  by  artificially  stimulating  them.  In  this  case,  how- 
ever, they  will  contract  without  any  reference  to  the  auricular 
beat.  Likewise  the  base  of  the  large  veins,  or  the  sinus  venosus 
as  this  is  known  in  the  amphibian  heart,  may  be  separated  from 
the  auricles  by  a  tight  thread.  The  auricles  now  continue  to 
beat,  but  at  a  much  slower  rate,  whereas  the  beat  of  the  sinus 
is  not  changed.  The  tissues  of  the  sinus  must  possess  to  a 
marked  degree  the  power  of  making  individual  or  automatic 
movements;  they  are  thus  able  to  control  the  rate  of  the  heart. 
For  this  reason  the  sinus  has  been  called  the  cardiac  pacemaker. 

The  great  muscular  development  of  the  human  heart  has 
caused  it  to  lose  some  of  its  primitive  characteristics.  Neverthe- 
less, there  still  exist  in  the  musculature  of  the  heart  some  strands 


THE  HEART  BEAT  165 

of  tissue  which  resemble  the  tissue  of  the  less  developed  or  more 
primitive  heart.  We  find  in  the  walls  of  the  auricles  small  nodes 
and  islets  of  tissue,  which  no  doubt  represent  the  sinus  tissues 
found  in  the  frog's  heart.  These  nodes  of  tissue  are  really  the 
pacemakers  of  the  heart,  for  it  is  in  them  that  the  impulse  or 
stimulus  arises  which  sets  agoing  the  contraction  of  the  auricles 
and  the  ventricles.  These  nodes  are  connected  by  fibers  wuth  the 
musculature  of  the  auricles  and  ventricles,  those  running  from 


Fig.  IS. — Dissection  of  heart  to  show  auriculo-ventricular  bundle  (Keith)  ; 
S,  the  beginning  of  the  bundle,  known  as  the  A-V  node ;  2,  the  bundle  dividing 
into  two  branches ;  i,  the  branch  running  on  the  right  side  of  the  interven- 
tricular   septum.       (From    Howell's    Physiology.) 

the  auricles  to  the  ventricles  being  gathered  into  a  bundle  of  tis- 
sue which  has  been  named  the  hundle  of  His  (Fig.  18). 

Numerous  cases  have  been  recorded  of  individuals  having  a 
very  irregular  or  a  very  slow  heart  beat  in  whom  post-mortem 
examination  of  the  heart  showed  a  diseased  condition  of  the 
bundle  of  His.  The  conditions  observed  in  man  have  been  re- 
produced in  the  case  of  animals  by  cutting  or  clamping  the  tis- 
sue about  this  bundle.  The  result  is  much  the  same  as  that  ob- 
served in  the  turtle's  heart  when  the  string  is  tied  between  the 
auricle  and  the  ventricle.  The  ventricle  may  continue  to  beat, 
but  it  does  so  without  reference  to  the  auricles.  Such  a  condi- 
tion is  known  as  Jieart  Mock. 


166  HUMAN  PHYSIOLOGY. 

It  is  of  interest  to  know  that  there  has  been  quite  an  advance 
recently  in  the  knowledge  of  the  conduction  of  the  cardiac  im- 
pulse from  the  auricles  on  to  the  ventricles.  It  has  been  known 
for  a  long  time  that  when  a  muscle  contracts,  a  small  but  definite 
electric  current  is  set  up  between  the  relaxed  and  the  contract- 
ing portions  of  the  muscles.  New  methods  of  detecting  and  re- 
cording the  direction  of  the  flow  of  such  currents  produced  in 
the  heart  in  man  have  shown  that  cases  of  heart  block  are  by  no 
means  rare.  The  instrument  used  for  this  purpose  is  a  highly 
sensitized  galvanometer,  and  the  tracings  are  known  as  electro- 
cardiograms. By  this  method  it  can  be  shown  that  in  certain 
cases  of  heart  disease  the  auricles  beat  twice  to  the  ventricles 
once,  or  again  that  the  auricles  may  beat  very  fast  while  the 
ventricles  are  beating  very  irregularly  and  slowly. 

The  Action  of  Inorganic  Salts  on  the  Heart  Beat. — A  very 
interesting  theory  has  recently  been  advanced  concerning  the 
cause  of  the  heart  beat.  It  will  be  remembered  that  the  blood 
contains  salts  of  sodium,  potassium  and  calcium  in  solution.  If 
these  salts  are  replaced  by  other  non-poisonous  salts  in  the  same 
concentration  as  the  salts  removed,  the  heart  will  not  beat.  If 
the  heart  is  perfused  with  a  solution  of  sodium  chloride  alone, 
the  beat  becomes  very  weak  and  finally  stops.  If,  however,  a 
small  amount  of  calcium  and  potassium  salts  is  added  to  the 
sodium  chloride  solution,  the  heart  will  again  begin  to  beat,  but  it 
stops  after  a  while  in  a  state  of  relaxation,  or  diastole,  if  calcium 
chloride  is  removed  from  the  solution,  or  in  systole,  or  contrac- 
tion, if  the  potassium  salts  are  removed.  These  experiments  sug- 
gest that  the  salts  of  the  blood  offer  a  solution  to  the  problem  of 
the  cause  of  the  heart  beat,  the  potassium  favoring  relaxation, 
and  the  calcium  contraction.  If  the  proper  balance  of  these 
salts  is  present  in  the  blood,  it  is  conceivable  that  a  regular  se- 
quence of  contraction  and  relaxation  of  cardiac  muscle  will  take 
place  because  of  the  action  of  the  salts. 

The  Vascular  Mechanism  of  the  Heart. 

Definition  of  Terms.-^A  definition  of  the  terms  applied  to 
the  different  phases  of  the  heart's  activity  will  help  in  the  de- 


THE   CARDIAC   CYCLE  167 

scription  of  the  events  which  occur  during  one  complete  heart 
beat.  The  period  of  actual  contraction  of  the  heart  is  termed 
systole.  This  is  divided  into  auricular  and  ventricular  systole. 
The  term  sphygmic  period  is  applied  to  that  part  of  ventricular 
systole  during  which  the  blood  is  actually  leaving  the  ventricles. 
The  period  of  relaxation  and  rest  of  the  cardiac  muscles  is  called 
diastole.  The  cardiac  cycle  includes  the  time  of  systole  and  dias- 
tole of  the  heart. 

The  Events  of  the  Cardiac  Cycle. — During  diastole  the  blood 
flows  in  a  steady  stream  from  the  great  veins  through  the  two 
auricles  into  the  ventricles,  the  auriculo-ventricular  valves  being 
open.  "When  the  ventricles  are  as  full  as  the  weight  and  the  pres- 
sure of  the  blood  can  make  them,  auricular  systole  begins.  The 
auriculo-ventricular  valves  at  this  instant  are  floating  in  the 
blood  which  has  collected  in  the  ventricles,  and  are  almost  in  the 
position  of  closure,  but  a  narrow  chink  still  remains  between 
them,  and  through  this,  auricular  systole  forces  blood  under 
pressure  into  the  ventricle,  thus  filling  the  ventricles  completely. 
At  the  dead  stop  of  auricular  systole  there  are  currents  of  blood 
reflected  back  along  the  sides  of  the  ventricles  which  strike  the 
under  surface  of  the  valves  and  completely  close  them.  Ven- 
tricular systole  now  begins.  The  closed  valves  prevent  the  pass- 
age of  blood  back  into  the  auricles,  and  the  entire  force  of  the 
ventricles  is  expended  in  forcing  the  blood  out  through  the  ar- 
terial openings.  Whenever  the  pressure  in  the  ventricles  exceeds 
that  in  the  arteries,  the  semilunar  valves  open  and  remain  open 
till  the  force  of  the  ventricle  falls  below  the  pressure  of  blood  in 
the  arteries.  The  time  between  the  closing  of  the  auriculo-ven- 
tricular valves  and  the  opening  of  the  semilunar  valves  is  called 
the  period  of  getting  up  power,  or  the  pre-sphygmic  period  (Fig. 
19). 

It  is  obvious  that  when  the  blood  is  leaving  the  ventricles  the 
pressure  must  be  less  in  the  arteries  than  in  the  heart.  Each  ven- 
tricle pours  out  more  blood  into  its  artery  than  can  pass  througli 
the  capillaries  in  the  same  unit  of  time,  and  hence  the  arterial 
walls  are  stretched  and  the  blood  is  put  under  their  elastic  ten- 
sion.   At  the  moment  the  ventricles  exert  less  pressure  than  does 


168 


HUMAN  PHYSIOLOGY. 

n 


the  elastic  recoil  of  the  arteries  on  the  blood,  the  semilunar  valves 
are  closed  tightly  by  backward  eddying  currents  in  the  arteries. 
Their  closure  prevents  any  return  of  blood  into  the  ventricles. 

The  blood,  having  attained  a  certain  momentum  during  the 
sphygmic  period,  is  carried  on  by  its  inertia  for  a  fraction  of  a 
second  after  the  ventricle  ceases  to  exert  pressure  on  it,  thus  pro- 
ducing a  partially  relaxed  artery  just  beyond  the  semilunar 
valves.  This  momentum  being  lost,  the  blood,  by  the  pressure 
which  the  stretched  elastic  wall  of  the  arteries  exerts  on  the 


Auricular 
£y6tole 

Ventricular     6yitole 

Pcxu.se                            1 

200 

ISO 

Ventricle 

/                                   -^ 

too 

^Aorko. 

ir 

":^^ 

SO 

i 

^          ^ — 

\     AuTidc^ 

\ 

. 

0 

\      / 

\     ^-— *'*' 

-a 
res 

T  C 

C  « 
T  - 
<< 

3 
3 

ISi 

Sound  of  heart 

Ss 

Sound  of  heart 

6( 

iconcls 

0.1                 |o.2               \o.3 

\o.U                \0.3                1 

Fig-.  19. — Diagram  showing-  relative  pressure  in  auricle,  ventricle  and  aorta. 

blood,  is  forced  back  on  to  the  semilunar  valves  and  into  the  par- 
tially relaxed  base  of  the  aorta.  The  blood,  being  thus  prevent- 
ed from  returning  to  the  heart,  must  continue  to  flow  on  into  the 
capillaries,  and  this  onward  flow  never  ceases,  because  the  next 
cardiac  systole  occurs  before  the  arteries  have  ceased  to  exert 
all  of  their  recoil  pressure  on  the  blood  (see  also  p.  173). 

After  the  arterial  valves  close,  the  ventricles  continue  to  relax, 
and  the  pressure  within  quickly  falls  below  that  which  obtains 


THE  HEART  SOUNDS  169 

in  the  partially  filled  auricles.  At  this  moment  the  weight  of  the 
blood  which  has  accumulated  in  the  auricles  during  the  systole, 
forces  the  valves  of  the  auriculo-ventricular  orifice  open,  and  the 
ventricle  again  begins  to  fill.  The  period  between  the  closure  of 
the  semilunar  valves  and  the  opening  of  the  auriculo-ventricular 
valves  is  known  as  the  post-sphygtnic  period,  and  is  the  begin- 
ning of  the  diastole  of  the  ventricles.  The  above  events  com- 
prise those  taking  place  in  a  complete  cardiac  cycle. 

The  Heart  Sounds. — If  one  applies  his  ear  to  the  front  of  the 
chest,  or  better  still  uses  a  stethoscope,  which  physicians  use  to 
examine  the  sounds  of"  the  lungs  and  heart,  two  sounds  will  be 
heard  during  each  cardiac  cycle.  The  first  sound  is  dull,  low 
pitched,  and  long ;  the  second  sharp,  high  and  short.  Following 
the  second  sound  is  a  short  pause.  It  has  been  determined  ex- 
perimentally that  the  first  sound  is  caused  partly  by  the  closure 
and  sudden  tension  of  the  auriculo-ventricular  valves  at  the  mo- 
ment of  cardiac  systole,  and  partly  by  the  muscular  contraction 
of  the  ventricle.  Anything  which  interferes  with  the  closure  of 
the  valves  causes  an  alteration  in  the  sound;  for  instance,  if  the 
valves  are  diseased  there  will  be  a  leaking  of  blood  back  into  the 
auricles  during  systole,  and  this  will  cause  a  distinct  murmur  to 
take  the  place  of  the  sound.  If  the  musculature  of  the  heart  is 
weakened,  the  sound  is  also  modified.  Hence  the  first  sound  of 
the  heart  is  an  important  diagnostic  sign  in  heart  disease.  The 
second  sound  of  the  heart  is  due  to  the  sudden  tension  exerted 
on  the  semilunar  valves  at  the  moment  the  blood  is  forced  back 
on  them,  following  ventricular  systole.  This  sound  is  also  sub- 
ject to  variations  in  heart  disease,  especially  in  disease  of  the 
valves  themselves,  in  which  case  because  of  roughening  they  may 
offer  resistance  to  the  outrush  of  blood  from  the  ventricles,  or  by 
jiot  closing  tightly,  allow  the  passage  of  blood  in  the  wrong  direc- 
tion. In  either  case  the  sound  is  changed  in  character  and  is  a 
useful  diagnostic  sign. 

By  using  these  heart  sounds  as  signals  of  the  events  occurring 
within  the  heart,  it  is  possible  to  calculate  the  time  relations  of 
the  various  phases  of  the  cardiac  cycle.  The  heart  in  the  ordinary 
individual  beats  about  seventy  times  a  minute,  so  that  we  may 


170  HUMAN  PHYSIOLOGY.     . 

say  that  the  cardiac  cycle  is  completed  in  about  eight-tenths  of  a 
second.  Systole  of  the  auricles  takes  about  one-tenth  of  a  second, 
systole  of  the  ventricle  three-tenths  of  a  second,  and  diastole 
about  five-tenths  of  a  second. 

Diseases  of  Cardiac  Valves.— If  the  mitral  valve  is  diseased, 
the  blood  may  be  retarded  from  flowing  from  the  auricle  into  the 
ventricle.  This  condition  is  called  mitral  stenosis.  If  the  valves 
cannot  close  tightly  and  thereby  permit  the  blood  to  regurgitate 
into  the  auricle  during  ventricular  systole,  the  condition  is  called 
mitral  insufficiency.  Disease  of  the  semilunar  valves  is  likewise 
divided  into  aortic  stenosis  and  insufficiency,  depending  on  the 
character  of  the  functional  change  in  the  valves. 


CHAPTER  XIX. 

THE  CIRCULATION  (Cont'd). 

The  Blood  Flow  Through  the  Vessels. 

Introduction. — A  clearer  idea  of  the  principles  governing  the 
circulation  of  blood  through  the  vessels  can  be  had  if  the  laws 
governing  the  flow  of  water  in  a  city  water  system  are  called  to 
mind.  For  example,  a  water-works  system  is  arranged  by  means 
of  either  special  pumps  or  a  standpipe,  to  furnish  a  stream  of 
water  at  a  constant  rate  and  pressure  into  the  city  water  mains. 
The  water  is  first  forced  into  one  large  pipe  and  from  this  de- 
livered to  the  consumer  by  means  of  much  smaller  pipes.  By 
simple  mathematical  calcul-ation  it  can  be  shown  that  the  total 
cross-section  area  of  the  smaller  pipes  is  many  times  that  of  the 
main  pipe,;  for  the  sake  of  argument,  let  us  say  800  times  great- 
er. Therefore  the  average  rate  of  flow  of  water  in  the  smaller 
pipes  must  be  800  times  less  than  in  the  main  pipe,  providing  all 
the  outlets  are  open.  However,  if  only  one-half  of  the  distribut- 
ing pipes  are  in  use,  the  flow  of  water  would  be  only  400  times 
less  than  in  the  main  pipe,  and  the  resistance  offered  by  the  walls 
of  the  pipes  to  the  flowing  water  is  also  halved.  Thus  the  same 
amount  of  water  is  delivered  in  the  same  unit  of  time  but  under 
twice  the  pressure,  since  only  one-half  of  the  force  used  to  deliv- 
er the  water  through  all  the  pipes  is  used  in  delivering  it  through 
one  half  of  them.  In  other  words,  it  takes  X  force  to  overcome 
the  resistance  offered  by  Y,  therefore  X  equals  Y.  When  X  re- 
mains constant  and  Y  is  halved,  then  X — Y/2  equals  X/2,  leav- 
ing X/2  as  a  remainder.  To  bring  it  home,  there  is  less  water 
delivered  from  the  garden  hose  and  it  has  far  less  pressure  be- 
hind it  when  all  the  neighbors  are  also  using  the  water,  than 
there  is  w^hen  only  a  few  outlets  are  in  use.  Likewise,  if  the 
amount  and  the  pressure  of  water  in  the  main  pipe  are  varied 
by  changing  the  force  of  the  pumps  or  the  level  of  water  in  the 
stand  pipe,  the  amount  and  pressure  of  water  delivered  are  also 
varied  in  the  same  direction. 

171 


172  HUMAN   PHYSIOLOGY. 

The  pumps  or  the  standpipe  correspond  to  the  heart  and  the 
large  arteries,  the  distributing  pipes  to  the  smaller  arteries  and 
capillaries.  With  these  ideas  in  mind  let  us  consider  the  part  the 
heart  and  blood  vessels  play  in  maintaining  the  circulation. 

The  Part  the  Heart  Plays. — At  each  systole  60  to  90  c.  c.  of 
blood  are  forced  into  the  aorta.  Cardiac  systole  lasts  about  0.3 
of  a  second,  the  diastole  0.5  second.  Therefore  the  heart  is  rest- 
ing about  60  per  cent  of  the  time.  By  experiment  it  has  been 
demonstrated  that  the  left  ventricle  forces  the  blood  out  into  the 
aorta  with  a  pressure  equivalent  to  the  weight  of  a  column  of 
mercury  from  160  to  190  mm.  in  height.  The  heart  alone,  how- 
ever, actually  propels  the  blood  through  the  arteries  for  only  the 
time  of  its  systole;  during  the  diastole,  as  already  explained,  the 
blood  would  cease  to  flow  entirely  if  it  were  not  for  the  part 
which  the  large  arteries  play  in  maintaining  the  circulation. 

The  Part  the  Arteries  Play. — If  100  c.  c.  of  water  are  forced 
in  0.3  second  into  an  ordinary  metal  pipe  at  intervals  of  0.8  of  a 
second,  100  c.  c.  must  flow  out  from  the  opposite  end  in  0.3  sec- 
ond ;  during  0.5  second  no  water  will  be  flowing  in  the  tube.  Let 
us  now  replace  the  metal  tube  with  an  elastic  rubber  tube,  the 
end  of  which  is  fitted  with  a  nozzle  filled  with  glass  beads.  Now 
if  100  c.  e.  of  water  are  forced  into  the  tube  in  0.3  second,  the 
rubber  tube  expands  because  the  beads  retard  the  free  outflow  of 
water  and  thus  make  it  impossible  for  100  c.  c.  of  water  to  pass 
through  them  in  the  time  allotted.  After  the  water  ceases  to  flow 
into  the  tube,  the  water  stored  up  in  the  expanded  portion  con- 
tinues to  flow  out  through  the  beads  because  of  the  elastic  recoil 
of  the  rubber.  If  the  resistance  offered  to  the  water  and  the 
expansile  force  of  the  tube  be  properly  adjusted,  a  constant 
stream  of  water  may  be  obtained  from  the  outlet,  in  spite  of  the 
fact  that  an  intermittent  force  is  supplying  the  water  (Fig.  20). 

The  intermittent  stream  of  the  arteries  is  changed  into  the 
constant  stream  in  the  veins  by  a  somewhat  similar  process.  The 
walls  of  the  arteries  are  composed  in  part  of  a  layer  of  strong 
elastic  tissue,  and  this  expands  to  a  greater  or  less  degree  at  each 
heart  beat,    The  resistance  which  the  arteries  and  the  capillaries 


ARTERIAL  BLOOD  PRESSURE.  173 

offer  to  the  flow  of  blood  prevents  the  passage  of  the  entire  sys- 
tolic output  of  the  heart  into  the  veins  during  the  actual  ven- 
tricular contraction.  It  is,  therefore,  necessary  that  the  large 
arteries  expand  in  order  to  make  room  for  the  blood.  A  part  of 
the  energy  of  the  heart  beat  is  stored  up  in  the  elastic  coats  of 
the  arteries,  and  after  closure  of  the  semilunar  valves,  which 
guard  the  ventricular  orifice,  the  blood  in  the  distended  arteries 
is  forced  on  through  the  capillaries  by  the  pressure  of  the  ar- 
terial walls. 

Arterial  Blood  Pressure. — From  the  foregoing  description  wfe 
see  that  there  are  several  factors  which  contribute  to  the  main- 


Fig.  20. — Diagram  of  experiment  to  show  how  a  pulse  (produced  by  com- 
pressing the  bulb  B)  comes  to  disappear  when  fluid  flows  through  an  elastic 
tube  (F)  when  there  is  resistance  (a)  to  the  outflow.  A,  basin  of  water; 
B,  biilb  syringe ;  C  and  E,  stop  cocks ;  D,  rigid  tube ;  P,  elastic  tube ;  G, 
bulb   flUed   with  sponge. 

tenance  of  a  constant  stream  of  blood  through  the  capillaries: 
viz.,  the  pumping  action  of  the  heart,  the  resistance  of  the  ar- 
terioles and  capillaries,  the  elastic  recoil  of  the  blood  vessels,  and 
the  amount  of  blood  itself.  That  the  velocity  and  the  pressure 
of  the  blood  depend  on  these  factors  was  first  of  all  demonstrated 
in  1732  by  Rev.  Stephen  Hales,  who  in  a  book  published  in  that 
year  reports  having  experimentally  determined  the  blood  pres- 
sure in  the  femoral  artery  of  a  horse.  He  found  that  the  pres- 
sure was  .sufficient  to  raise  the  blood  in  a  tube  seven  feet  above 
the  level  of  the  heart,  and  he  also  observed  that  each  beat  of  the 
heart  and  each  respiratory  movement  affected  the  pressure  of  the 
blood.  The  pressure  exerted  by  the  blood  on  the  vessel  wall  at 
the  height  of  the  systole  of  the  ventricle  is  known  as  the  systolic 
hlood  pressure,  and  that  exerted  by  the  ela.stic  recoil  of  the 
arteries  on  the  blood  during  the  diastole  of  the  heart  is  known 


174 


HUMAN  PHYSIOLOGY. 


as  the  diastolic  Mood  pressure.    The  average  between  these  two 
pressures  is  called  the  average  or  mean  arterial  blood  pressure. 
Since  Hales'  experiment  better  apparatus  has  been  devised  to 


Fig.  21. — Apparatus  for  taking  a  tracing  of  the  blood  pressure. 


measure  the  blood  pressure  in  animals  under  different  conditions. 
The  standard  method  consists  in  placing  a  tube,  called  a  cannula, 
directly  into  a  blood  vessel.  This  is  connected  with  a  rubber 
tube  filled  with  an  anti-clotting  mixture  (see  Fig.  21)  with  one 
arm  of  a  U  tube  partly  filled  with  mercury.  "When  the  blood  ves- 
sel is  opened,  the  pressure  of  the  blood  will  force  the  mercury 


ARTERIAL  BLOOD  PRESSURE.  175 

down  in  one  arm  and  up  in  the  other  arm  of  the  U  tube.  The 
difference  between  the  levels  of  the  mercury  in  the  two  arms  mul- 
tiplied by  13.5,  the  specific  gravity  of  mercury,  gives  the  pres- 
sure of  the  blood  in  terms  of  water,  or,  as  is  usually  done,  the 
blood  pressure  is  expressed  as  the  number  of  millimeters  through 
which  the  mercury  has  been  raised. 

Determinations  of  the  pressure  existing  in  different  portions 
of  the  vascular  system  show  that  there  is  a  steady  decrease  of 
pressure  of  the  blood  from  the  aorta  to  the  entrance  of  the  vena 
cava  into  the  right  auricle.  It  thus  happens  that  the  blood  is  al- 
ways flowing  from  a  place  of  higher  pressure  to  one  of  lower 
pressure. 

Methods  which  are  of  much  practical  importance  in  the  diag- 
nosis of  vascular  diseases  have  been  devised  to  determine  the 
blood  pressure  in  man.  The  principle  of  these  methods  consists 
in  measuring  the  pressure  required  to  shut  off  completely  the 
blood  supply  in  an  artery.  This  is  accomplished  by  placing  a 
rubber  sac  encased  in  a  leather  band  about  the  arm  (Fig.  22). 
By  means  of  tubing  this  sac  is  connected  with  a  mercury  gauge 
and  an  air  pump.  When  the  sac  is  pumped  up  with  air,  the  ves- 
sels in  the  arm  are  compressed,  and  when  the  blood  can  no  longer 
force  its  way  under  the  obstruction,  the  pulse  at  the  wrist  disap- 
pears and  at  this  moment  the  height  of  the  mercury  in  the  gauge 
is  measured.  This  represents  the  systolic  blood  pressure.  If  de- 
sired, a  similar  measurement  may  be  made  in  the  arteries  of  the 
leg. 

To  measure  the  diastolic  pressure  is  more  difficult.'  The  method 
depends  on  the  experimentally  determined  fact  that  when  the 
pulse  wave  produced  in  the  arteries  by  each  systole  of  the  heart, 
is  of  greatest  amplitude,  the  pressure  in  the  air  sac  or  compress- 
ing band  equals  the  lowest  pressure  present  in  the  vessel  between 
the  pulses. 

Recently  improvements  have  been  made  in  the  method  of  judg- 
ing the  point  of  obliteration  of  the  artery,  and  also  the  point  of 
maximum  pulsation,  by  listening  to  the  sounds  produced  at  each 
pulse  wave  when  the  artery  is  being  compressed. 

The  systolic  blood  pressure  in  the  artery  of  the  arm  in  healthy 


176 


HUMAN   PHYSIOLOGY. 


young  men  varies  from  110  to  130  mm.  of  mercury  when  it  is 
determined  in  the  sitting  posture.  When  a  person  is  lying  down 
the  pressure  is  a  little  less,  and  after  hard  exercise  a  little  higher. 
The  blood  pressure  under  ordinary  conditions  is  relatively  con- 
stant, and  is  dependent  on  a  delicate  adjustment  of  the  relation- 
ship existing  between  the  force  of  the  heart,  the  amount  of  blood 


Fig.  22.- — Apparatus  for  measuring  the  arterial  blood  pressure  in  man. 
The  pressure  in  the  cuff  is  raised  by  means  of  the  syringe  until  the  pulse 
can  no  longer  be  felt  at  the  wrist.  This  pressure  is  read  off  on  the  mercury 
manometer    (systolic  pressure). 


pumped  at  each  beat,  the  resistance  which  the  walls  of  the  blood 
vessels  offer  to  the  flow  of  the  blood,  the  size  of  the  vascular  sys- 
tem, and  the  amount  of  blood  in  the  body.  Since  the  amount  of 
blood  in  the  body  is  relatively  constant,  we  may  say  that  the 
factors  which  change  are  the  heart  and  the  blood  vessels.    How 


THE  VELOCITY  OP  THE  BLOOD  177 

these  factors  influence  the  blood  pressure  may  be  seen  if  we  again 
compare  the  system  to  the  city  water  supjoly. 

Factors  Which  Maintain  Blood  Pressure. — When  the  most 
water  is  being  pumped  into  the  mains,  then  the  water  has  the 
greatest  velocity  and  pressure.  Likewise,  when  the  heart  is 
pumping  most  blood  into  the  aorta,  the  velocity  and  the  pressure 
of  blood  in  the  vessels  are  the  greatest.  If  the  amount  of  water 
remains  constant,  a  uniform  outflow  through  all  the  outlet  tubes 
will  be  maintained,  but  if  the  number  of  outlet  tubes  be  dimin- 
ished, then  more  water  will  have  to  flow,  per  minute  of  time, 
through  the  remaining  tubes;  hence  the  velocity  and  the  pres- 
sure must  be  increased. 

The  same  conditions  are  present  in  the  body.  A  narrowing  of 
the  arterioles  throughout  the  body  or  in  some  extensive  vascular 
area,  causes  the  pressure  and  the  velocity  of  the  blood  to  be  in- 
creased in  the  remaining  vessels,  provided,  of  course,  the  heart 
beat  is  unchanged.  A  dilation  of  the  arterioles,  on  the  other 
hand,  results  in  a  fall  of  pressure  and  a  decrease  in  the  velocity 
of  the  blood.  In  the  same  way  also  an  increase  or  decrease  in  the 
action  of  the  heart  will  result  in  an  increase  or  decrease  in  the 
pressure  and  velocity  of  the  blood. 

The  dependence  of  these  two  factors,  i.  e.,  the  heart  and  the 
vascular  system,  on  the  maintenance  of  the  normal  blood  pres- 
sure, is  seen  in  the  fact  that,  with  a  fast  heart  and  dilated  blood 
vessels,  the  blood  pressure  may  be  exactly  the  same  as  when  the 
heart  is  beating  very  slowly  but  the  arterioles  are  all  constricted. 
It  is  apparent,  therefore,  that  the  velocity  of  the  blood  in  the 
vessels  is  dependent  on  the  pressure  of  the  blood  and  the  extent 
of  the  vascular  area  at  the  time  in  question. 

The  Velocity  of  the  Blood.— By  the  velocity  of  the  flow  of 
blood  we  mean  the  actual  time  it  takes  for  a  particle  of  blood  to 
pass  between  two  points.  If  the  rate  were  uniform  throughout 
the  vascular  area,  we  could  compute  the  time  which  a  particle  of 
blood  would  take  to  pass  through  the  circulatory  system.  This 
is  not  the  case,  however,  for  the  flow  of  blood  is  much  swifter  in 
the  aorta  than  in  the  smaller  vessels,  and  here  again  our  analogy 
between  the  circulatory  system  and  the  city  water  system  applies. 


178  HUMAN   PHYSIOLOGY. 

Just  as  the  combined  cross  area  of  the  small  pipes  leading  from 
the  main  pipe  of  the  water  system  is  greater  by  many  times  than 
the  area  of  the  main  pipe,  so  it  has  been  estimated  that  the  total 
cross  section  of  the  capillaries  of  the  body  is  800  times  larger 
than  that  of  the  aorta. 

It  has  been  estimated  that  the  rate  of  blood  flow  in  the  aorta  is 
about  320  mm.  per  second.  The  average  rate  of  flow  in  the  capil- 
laries must  then  be  800  times  less  than  that  in  the  aorta,  or  0.4 
mm.  per  second.  As  the  length  of  a  capillary  has  been  estimated 
to  be  about  0.5  mm.,  the  blood  takes  about  a  second  to  pass 
through  them  into  the  veins.  This  l;ias  been  verified  by  micro- 
scopic examination  of  the  blood  flow  in  the  capillaries. 

The  velocity  of  the  blood  must  be  altered  whenever  the  size  of 
the  vascular  area  is  changed,  and  since  during  a  cardiac  cycle 
exactly  the  same  amount  of  blood  is  delivered  into  the  right 
auricle  as  the  left  ventricle  forces  out  into  the  aorta,  it  follows 
that  the  same  amount  must  pass  through  the  vascular  area  of  the 
body  in  the  same  time.  In  other  words,  the  amount  of  blood 
which  flows  in  a  given  series  of  blood  vessels  in  a  given  time  is  in- 
dependent of  the  size  of  the  blood  vessels. 

The  Return  of  the  Blood  to  the  Heart. — We  must  now  con- 
sider the  nature  of  the  force  which  propels  the  blood,  and  study 
what  changes  take  place  in  the  movement  of  the  blood  during  its 
passage  through  the  vessels. 

The  blood  is  expelled  from  the  left  ventricle  with  consider- 
able force  and  at  a  high  velocity.  On  its  way  through  the  body 
much  of  the  energy  given  out  by  the  contraction  of  the  heart  is 
used  to  overcome  the  resistance  offered  by  the  walls  of  the  ves- 
sels and  the  capillaries.  In  consequence  of  this,  the  velocity  and 
the  pressure  of  the  blood  on  the  sides  of  the  vessels  are  much  re- 
duced. 

The  blood  is  collected  from  the  capillaries  by  the  veins,  and 
since  the  volume  of  the  veins  is  less  than  the  volume  of  the  capil- 
laries its  velocity  is  much  increased.  The  relatively  large  caliber 
of  the  veins,  however,  offers  little  resistance  to  the  flow  of  blood, 
and  the  energy  remaining  from  that  imparted  to  the  blood  by  the 
heart  has  full  power  to  make. itself  felt.    Nevertheless,  this  is  not 


THE  CIRCULATION  TIME.  179 

sufficient  alone  to  force  the  blood  onward  and  back  to  the  heart, 
and  we  must  seek  other  accessory  factors  to  explain  the  venous 
return. 

The  veins  are  ecjuipped  with  cup-shaped  valves  which  permit 
the  passage  of  blood  only  in  one  direction,  i.  e.,  towards  the  heart. 
Every  movement  of  a  muscle  therefore  squeezes  some  of  the  blood 
onward.  This  massaging  influence  of  the  muscles  is  very  im- 
portant. Its  absence  accounts  for  the  fact  that  it  is  impossible 
to  stand  still  for  a  long  period  of  time  without  the  limbs  becom- 
ing very  painful,  especially  in  the  case  of  varicose  veins,  where 
the  valves  of  the  veins  are  no  longer  functional,  so  that  there  is 
nothing  to  prevent  the  blood  from  returning  to  the  more  depend- 
ent positions.  Another  source  of  energy  to  the  returning  blood 
is  the  aspiratory  effect  of  the  thorax  at  each  inspiration.  This 
action  will  be  considered  in  the  study  of  the  respiratory  mechan- 
ism. 

Circulation  Time. — The  actual  time  which  is  taken  for  the 
blood  to  traverse  the  circulatory  system  has  been  variously  esti- 
mated. Obviously  such  figures  can  give  only  average  results, 
since  the  distance  through  which  blood  to  the  arm  must  flow  is 
less  than  that  to  the  legs.  In  general,  it  may  be  said  that  the 
blood  makes  a  complete  circulation  in  from  25  to  30  beats  of  the 
heart.  The  circulation  through  the  lungs  requires  about  one- 
fourth  of  this  time. 

That  the  velocity  of  the  blood  flow  through  different  vessels 
varies,  is  apparent  from  actual  observations  made  on  severing 
them  and  actually  observing  the  rate  of  outflow.  The  following 
figures  expressing  the  hlood  supply  per  minute  to  each  hundred 
grants  of  organ  have  been  determined  experimentally: 

Leg    5  c.  c.  Liver    (venous) ....   59  c.  c. 

Head    20    "  Liver    (arterial) ...  25    " 

Stomach 21    "  Brain    136    " 

Intestines  .  . .  .31    "  Kidney 150    " 

Spleen    58    "  Thyroid   560    " 

The  Effect  of  the  Circulation  on  the  Blood. — If  the  circula- 
tion of  the  blood  through  the  vessels  of  the  lung  or  the  web  of  a 


180  HUMAN   PHYSIOLOGY. 

frog's  foot  be  examined  by  means  of  a  microscope,  several  inter- 
esting facts  will  be  noted.  The  red  blood  corpuscles  will  be  seen 
flowing  in  the  center  of  the  blood  vessels,  while  in  the  clear  plas- 
ma which  surrounds  them  are  the  much  less  numerous  white 
cells.  This  arrangement  is  explained  by  the  fact  that  the  red 
corpuscles  are  heavier  than  the  white  cells  or  the  plasma  and  are 
held  in  the  center  of  the  stream  by  a  principle  of  hydraulics.  The 
white  cells  flow  more  slowly  along  the  sides  of  the  vessels  than 
the  red  corpuscles  do  in  the  center  of  the  stream,  which  is  sug- 
gestive of  the  function  of  the  white  cells  as  phagocytes  (see  p. 
152)  ;  thus,  any  injury  to  the  vessel  wall  will  necessarily  slow  the 
flow  of  blood  through  the  veins  and  allow  a  greater  number  of 
leucocytes  to  collect  at  the  point  of  injury. 

The  Pulsatile  Acceleration  of  Blood  Flow. — The  flow  of  blood 
in  the  arteries  differs  from  that  in  the  veins  and  the  capillaries 
in  that  it  is  swifter  and  pulsatile  in  character.  This  pulsatile 
variation  is  due  to  the  acceleration  of  the  blood  flow  caused  by 
each  heart  beat,  and  the  reason  that  this  is  not  seen  in  the  capil- 
laries and  veins  is  that  the  resistance  which  the  walls  of  the 
capillaries  and  arterioles  offer  to  the  blood  is  so  great  that  the 
cardiac  factor,  acting  only  for  a  brief  time,  is  lost.  The  energy 
represented  in  the  increased  rate  of  flow,  is  spent  in  stretching  the 
walls  of  the  arteries,  which  contract  after  the  pulsatile  wave  has 
passed,  and  thus  force  the  blood  onward. 

The  Pulse. — The  pulsatile  expansion  of  the  arteries  at  each 
heart  beat  has  been  mentioned  in  connection  with  the  factors 
which  help  to  maintain  the  normal  blood  pressure.  It  is  this 
also  which  produces  the  phenomenon  which  is  known  as  the  pulse. 
From  time  immemorial  the  physician  has  been  accustomed  to 
come  to  an  idea  concerning  the  condition  of  the  circulation  by 
feeling  the  pulse,  for  it  represents  changes  in  the  arterial  ten- 
sion occurring  during  each  cardiac  cycle.  In  order  to  study 
the  pulse  wave  more  carefully,  instruments  have  been  devised 
which  graphically  record  its  wave  on  a  piece  of  paper.  Such  an 
instrument*  is  known  as  a  sphygmograph  (Fig.  23),  and  some 
of  these  have  been  cleverly  arranged  so  as  to  enable  us  to  record 
simultaneously  the  pulse  from  different  blood  vessels. 


THE  PULSE. 


181 


Since  the  pulse  is  not  due  to  an  actual  movement  of  blood 
along  the  arteries,  but  rather  to  changes  in  tension  producing 
an  expansion  of  the  vessel  wall,  it  follows  that  the  transmission 
of  the  wave  may  be  much  more  rapid  than  the  movement  of 
blood.  This  may  be  explained  by  reference  to  the  motion  im- 
parted to  a  row  of  billiard  balls  when  the  one  on  the  end  is  hit 
with  the  cue.  The  one  hit  actually  moves  very  little,  but  imparts 
its  energy  of  movement  to  the  others,  so  that  the  ball  at  the  end 
of  the  row  moves  away  with  some  velocity,  while  the  others  move 
slowly.     The  wave  of  energy  spreads  in  a  fraction  of  a  second 


8  <^ 

Fig.   23. — Jaquet  Sphygmocardiograph. 


from  ball  to  ball.  By  simultaneously  taking  tracings  of  the  caro- 
tid and  the  radial  pulses,  for  example,  it  has  been  computed  that 
the  pulse  wave  is  transmitted  at  the  rate  of  ten  meters  a  second, 
and  that  it  may  be  six  meters  long.  This  means  that  the  pulse 
wave  reaches  the  peripheral  vessels  before  the  systole  of  the 
heart  is  completed.  Any  local  change  in  the  vessel  may  slow 
down  the  rate  of  transmission,  and  if  there  is  a  difference  in  the 
appearance  of  the  pulse  in  the  two  arms  or  legs,  it  is  indicative 
of  some  obstruction  or  change  in  one  of  the  vessels.  * 

When  we  analyze  the  pulse  wave  obtained  by  a  sphymograph 
taken,  for  example,  from  the  radial  artery,  it  is  seen  that  the 


182  HUMAN  PHYSIOLOGY. 

first  elevation  is  very  rapid  and  abrupt  (Fig.  24,  a).  This  is 
caused  by  the  sudden  increase  in  pressure  of  the  blood,  due  to 
cardiac  systole,  resulting  in  the  sudden  expansion  of  the  artery. 
Following  the  abrupt  rise,  the  curve  gradually  descends  till  the 
next  heart  beat  occurs.  During  this  period  the  arterial  blood 
pressure  is  maintained  by  the  elastic  recoil  of  the  stretched 
arteries.  On  the  descending  curve  there  are  as  a  rule  several  small 
waves  and  depressions.  Of  these  waves  the  large  one  (Fig. 
24,  b)  is  always  present  and  is  known  as  the  dicrotic  wave,  and 
the  dicrotic  notch  is  the  depression  immediately  preceding  the 
wave.  The  presence  of  this  wave  is  explained  as  follows:  At 
the  end  of  cardiac  systole  the  blood,  under  the  influence  of  the 
pressure  exerted  by  the  stretched  walls  of  the  arteries,  is  forced 
both  towards  the  peripheral  vessels  and  back  towards  the  heart. 


Fi)?.  24. — Pulse  tracing  made  by  sphygmograph.  A,  systolic  wave;  B, 
dicrotic  wave. 

The  cardiac  semilunar  valves,  being  tightly  closed  at  the  end 
of  systole,  arrest  the  back  flowing  blood,  and  it  rebounds,  as  it 
were,  producing  the  depression  with  the  wave  which  is  re- 
flected over  the  entire  circulation.  When  the  blood  pressure  is 
high,  the  secondary  waves  make  very  little  depression,  because  of 
their  relatively  low  pressure,  but  in  conditions  where  the  blood 
pressure  is  low,  as  in  typhoid  fever,  surgical  shock,  a  faint,  and 
in  deep  anesthesia,  the  dicrotic  wave  is  easily  felt  by  the  finger. 

Other  qualities  of  the  pulse  which  may  assist  the  physician  in 
judging  of  the  condition  of  the  circulatory  system  are  its  rate 
and  its  compressibility.  Its  rate  tells  us  how  fast  the  heart  is 
beating,  and  its  compressibility  gives  a  rough  idea  of  the  blood 
pressure. 

The  Circulation  Through  the  Lungs. — In  general  the  same 
conditions  are  present  in  the  circulation  of  the  blood  through 


THE    PULMONARY    CIRCULATION.  183 

the  lungs  as  are  found  in  the  systemic  circulation.  The  right 
ventricle  is  far  less  powerful  than  the  left,  so  that  the  pressure 
of  the  blood  in  the  lung  vessels  is  less  than  that  in  the  systemic 
vessels.  The  respiratory  movements  also  cause  the  size  of  the 
blood  vessels  in  the  lungs  to  vary  in  a  marked  degree.  These 
changes  in  the  capacity  of  the  pulmonary  blood  vessels  affect  the 
systemic  blood  pressure.  Thus,  at  the  height  of  inspiration,  the 
lungs  may  contain  one-twelfth  of  the  blood  of  the  body,  while 
during  expiration  this  amount  may  be  lessened  to  one-fifteenth 
to  one-eighteenth  of  the  total.  This  condition  makes  it  possible 
for  the  heart  to  be  filled  more  rapidly  during  the  later  part  of 
inspiration  and  the  beginning  of  expiration,  than  at  other  times, 
and  accounts  for  the  rise  of  blood  pressure  observed  at  this  time. 


CHAPTER  XX. 

THE  CIRCULATION  (Cont'd). 

The  Influence  of  the  Nervous  System  on  the  Circulation. 

Up  to  the  present  time  we  have  considered  the  circulatory 
system  as  a  purely  automatic  and  mechanical  apparatus  for 
carrying  blood  to  all  parts  of  the  body.  It  is  necessary  that 
this  apparatus  vary  in  its  activity,  not  only  according  to  the 
needs  of  the  body  as  a  whole,  but  also  according  to  the  needs  of 
the  various  parts  of  the  body.  It  would  be  poor  economy  for  the 
heart  to  maintain  through  all  parts  of  the  body  at  all  times  a 
stream  of  blood  which  would  be  large  enough  for  all  emergencies. 
There  must  be  some  way  of  controlling  the  blood  flow  according 
to  the  needs  of  the  body.  This  function  is  served  primarily  by 
the  central  nervous  system,  which  is  connected  by  means  of 
nerves  with  the  musculature  of  the  heart  and  the  blood  vessels, 
and  secondarily  by  secretions  from  the  so-called  ductless  glands, 
the  best  known  of  which  are  the  adrenal  glands  (see  p.  129). 

The  Nervous  Ocntrol  of  the  Heart. 

The  Cardiac  Nerves. — The  heart  is  supplied  with  both  sen- 
sory and  motor  nerves.  Sensory  nerves  carry  stimuli  from  the 
peripheral  regions  to  the  brain  and  are  known  as  afferent  nerves. 
Motor  nerves,  on  the  other  hand,  carry  stimuli  from  the  brain 
to  the  muscles  or  glands,  and  are  known  as  efferent  nerves.  The 
efferent  nerves  of  the  heart  are  found  in  fibers  coming  from  the 
spinal  cord  by  way  of  the  sympathetic  system,  and  by  the  vagi 
or  the  tenth  pair  of  cranial  nerves  (see  p.  265).  It  must  be 
clearly  understood  that  the  nerves  merely  regulate  the  heart 
beat,  but  have  nothing  to  do  with  its  occurrence.  In  other  words, 
the  heart  continues  to  beat  after  all  the  nerves  have  been  severed. 

The  Accelerator  Nerves. — To  understand  how  the  fibers 
reach  the  heart,  the  reader  is  referred  to  the  general  description 

184 


THE  CARDIAC  NERVES,  185 

of  the  S3mipathetic  nervous  s^^stem  on  page  277.  The  sympathetic 
fibers  of  the  heart  are  found  in  the  first  and  second  spinal  nerves 
of  the  thoracic  region.  After  connecting  with  nerve  cells  situ- 
ated in  the  stellate  ganglion,  they  go  to  the  heart,  where  they 
end  about  the  cardiac  muscular  fibers. 

Cutting  the  sympathetic  fibers  to  the  heart  causes  a  slower  beat 
and  a  prolonged  diastole.  On  the  other  hand,  stimulation  of  the 
nerves  with  an  electric  current  increases  the  rate  of  the  heart 
(Fig.  25).  For  the  above  reasons  the  sympathetic  nerves  to  the 
heart  are  known  as  accelerator  or  augment ory  nerves. 

The  Inhibitory  Nerves.— The  vagi  are  a  pair  of  nerves  arising 
on  each  side  of  the  medulla,  and  running  a  course  downwards 
through  the  neck  into  the  thoracic  and  abdominal  cavities.  This 
pair  of  nerves  supply  fibers  to  the  various  organs  of  these  regions 


Time  in  icconds 


normal  Vent  ricul  1*5  bimulotion  of    I     Normal        I  Stimulation  of  I      n^, ,.1 

larbeot  I  Vagus  nerve    I     f^''^"^^'        Uympothctic  F     ^0^"^*^' 

Fig.  25.— Effect  of  stimulating  vagus  and  sympathetic  nerves  on  the  frog's 
heart. 

including  tlie  heart,  which  receives  branches  from  both  vagi. 
It  is  possible  by  simple  experiments  to  demonstrate  the  function 
of  these  fibers. 

For  example,  if  the  vagus  on  one  side  be  cut,  the  heart  rate 
will  increase  a  little ;  if  both  vagi  be  cut,  the  beat  is  still 
more  markedly  quickened,  and  the  increased  discharge  of  blood 
from  the  heart  produces  a  rise  in  the  arterial  blood  pressure 
(Fig.  26,  No.  III).  By  cutting  these  nerves  we  remove  the  influ- 
ence which  the  central  nervous  system  exerts  through  them  on 
the  heart  rate.  Since  the  heart  beats  faster  after  this  operation, 
we  must  conclude  that  this  organ  constantly  receives  stimuli 
from  the  brain  through  the  vagi,  and  that  these  stimuli  cause 


186 


HUMx\N  PHYSIOLOGY. 


No.I 


Tirnc  in  ieconds 


v-i  /'\/  \  «idney  Volume 


Time   in  seconds 


NoH 


Kidney  volutTife 


Blood  pressure  ,        j,. 


nerve   Cut 


Blood  pressure 

'(njection 
of  dilute 
odrenolin 
solution. 


Ham 


Time  in  seconds 


Time  m  Seconds 


NoEZ 


Kidney  Volumt: 


•Roptd   Tjleedin^     'Slow  bleeding 


Time  in  seconds 


No.Y 

Fig.  26. — Tracings  of  arterial  blood  pressure   (taken  with  apparatus  in  Fig. 
21)  and  of  kidney  volume  (taken  with  volume  recorder)  showing  the  effect  of: 
I.      Stimulation  of  the  vagus  nerve. 
II.     Stimulation  of  the  splanchnic  nerve. 

III.  Cutting  one  vagus  nerve. 

IV.  Injection  of  epinephrin   (adrenalin). 
V.     Hsemorrhage. 

The  tracings  all  read  ^rom  right  to  left. 


THE    CARDIAC    NERVES.  187 

the  heart  to  beat  more  slowly.  Such  a  continued  action  of  a  nerve 
is  known  as  a  tonic  influence. 

That  the  vagi  can  slow  the  heart  or  even  stop  it  altogether  is 
shown  by  stimulation  of  these  nerves  with  an  electric  current  of 
suitable  strength  (Fig.  25).  If  weak  shocks  are  employed,  the 
heart  is  slowed,  the  blood  pressure  falls  somewhat,  and  the 
diastolic  pressure  becomes  markedly  decreased,  because  the  ar- 
teries have  a  greater  period  of  time  in  which  to  empty  between 
the  beats.  If  somewhat  stronger  stimuli  be  used,  the  heart  will 
stop  beating  entirely,  and  remain  in  the  diastolic  position  for 
several  seconds,  during  which  the  blood  pressure  will  sink  to 
zero  (Fig.  26.  No.  I).  It  is  scarcely  possible  to  kill  an  animal 
by  stimulation  of  the  vagus,  however,  since  the  heart  will  begin 
to  beat  after  a  short  time  in  spite  of  the  continued  vagus  stimu- 
lation. This  phenomenon  is  known  as  escapement.  The  time  of 
its  onset  varies  considerably  in  different  animals.  It  has  been 
suggested  that  the  vagi  have  much  more  effect  on  the  auricles 
than  on  the  ventricles,  which  is  suggestive  of  the  auricles  being 
the  pacemakers  of  the  heart. 

Relation  of  the  Sympathetic  and  Vagus  Nerves  to  the  Heart. 
— The  antagonistic  action  existing  between  the  cardiac  fibers  of 
the  sympathetic  and  vagus  nerves  allows  the  heart  to  respond 
quickly  to  any  need  that  the  body  may  demand  of  it.  These 
demands  are  made  through  the  brain,  by  various  afferent  or  sen- 
sory nerves.    This  is  brought  about  in  the  following  way : 

The  Cardiac  Center. — In  the  medulla,  the  hind  part  of  the 
brain,  there  is  a  collection  of  nerve  cells  from  which  the  cardiac 
branches  of  the  vagus  arise.  Near  by  also  are  located  the  cells 
from  which  the  sympathetic  nerves  of  the  heart  arise.  Both  of 
these  nerve  centers,  for  by  this  term  are  known  the  important 
cell  stations  of  the  brain,  are  supplied  by  extensive  connections 
with  afferent  or  sensory  fibers  coming  from  all  parts  of  the  body, 
the  brain  and  even  the  heart.  The  centers  become  more  or  less 
active  in  responst;  to  impulses  reaching  them  along  the  sensory 
fibers. 

The  Cardiac  Depressor  Nerves. — One  of  the  most  important 
of  the  different  cardiac  nerves  is  that  known  as  the  cardiac  depres- 


188  HUMAN   PHYSIOLOGY. 

sor.  It  has  its  beginnings  in  filaments  lying  in  the  left  ventricle 
and  in  the  aorta,  and  runs  to  the  medulla  in  the  vagus  trunk  in 
most  mammals,  or  as  a  separate  nerve  in  the  rabbit.  Under  ordi- 
nary conditions,  cutting  this  nerve  produces  no  effect  on  the 
heart  beat,  but  stimulation  of  the  upper  end  of  the  cut  nerve, 
i.  e.,  the  end  running  to  the  head,  results  in  a  marked  slowing  of 
the  heart  and  fall  in  the  blood  pressure.  If  the  experiment  is 
repeated  after  cutting  the  vagi,  the  heart  is  slowed,  but  the  fall 
in  blood  pressure,  though  less  evident,  still  occurs.  The  normal 
stimulus  to  the  depressor  nerve  is  a  high  blood  pressure  in  the 
ventricles  and  aorta.  The  stipiulus,  thus  set  up,  acts  through  the 
vagus  center  and  the  vagus  nerve,  and  slows  the  heart.  It  also 
acts  on  the  vasomotor  center  and  causes  the  blood  vessels  to 
dilate.  Both  changes  produce  a  fall  in  the  blood  pressure.  The 
vagus  nerve,  besides  the  afferent  vagus  fibers,  carries  afferent  or 
sensory  nerves  to  the  vagus  center.  This  can  be  demonstrated 
by  cutting  one  vagus  and  stimulating  the  central  end,  i.  e.,  the 
end  running  to  the  brain.  A  marked  slowing  of  the  heart  usually 
results.  By  acting  through  the  vagus  center  and  nerves,  or 
through  the  sympathetic  center  and  nerves,  most  of  the  sensory 
nerves  of  the  body,  if  stimulated,  can  produce  a  reflex  slowing 
or  quickening  of  the  heart  beat.  One  cannot,  however,  always 
predict  exactly  what  result  will  be  obtained.  The  stimulation  of 
the  fifth  nerve  in  the  nasal  cavity  or  in  the  mouth  always  causes 
a  reflex  slowing  of  the  heart.  Stimulation  of  the  laryngeal  nerve 
and  the  nerves  of  the  peritoneum  have  a  similar  effect.  It 
is  also  of  interest  to  note  that  the  act  of  swallowing  will  often 
cause  a  decrease  in  the  rate  of  the  heart  through  reflex  vagus 
action. 

The  relation  of  the  blood  pressure  to  the  rate  of  the  heart  has 
been  noted  in  connection  with  the  cardiac  depressor  nerve 
(p.  187).  Anything  which  produces  an  increase  in  the  pulse 
rate,  other  conditions  being  equal,  will  cause  an  increase  in  the 
blood  pressure,  and  this  acts  reflexly  to  bring  about  a  slowing 
of  the  heart.  The  reverse  of  this  is  likewise  true.  In  this  quick- 
ening or  slowing  of  the  heart,  the  vagi  and  the  sympathetic 
nerves  always  act.     In  the  adult  the  normal  rate  of  the  heart 


THE   VASOMOTOR   NERVES,  189 

varies  between  68  and  76  per  minute.    In  children  the  rate  is  a 
little  faster,  and  in  infants  it  may  be  normally  130  or  more. 

The  Nervous  Control  of  the  Blood  Vessels. 

During  muscular  activity  the  metabolism  of  the  body  may  be 
increased  five  or  six  times,  as  can  be  judged  from  the  amount  of 
carbon  dioxide  given  off  by  the  lungs.  Since  this  increase  is  due 
to  the  activity  of  the  muscles,  it  is  necessary  that  these  obtain 
a  greater  supply  of  oxygen,  and  that  they  be  able  to  rid  them- 
selves of  the  carbon  dioxide  which  is  a  waste  product  of  their 
activity.  Every  other  organ  requires  an  increased  blood  supply 
when  it  becomes  active,  so  that  blood  has  to  be  diverted  from  the 
inactive  to  the  active  tissues,  and  the  least  important  activities 
of  the  body  have  to  be  subordinated  to  the  one  which  is  most 
needed  at  the  time  in  question.  This  action  is  brought  about 
partly  by  the  central  nervous  system,  acting  through  its  afferent 
and  efferent  nerves  on  the  musculature  of  the  blood  vessels  of 
the  body,  and  partly  by  means  of  chemical  substances  which  are 
produced  at  an  early  stage  of  the  activity  itself. 

The  Vasomotor  Nerves. — It  was  discovered  in  the  middle  of 
the  past  century  by  the  French  physiologist,  Claude  Bernard, 
that  section  of  the  cervical  sympathetic  nerve  in  the  neck  of  the 
rabbit  causes  a  marked  dilation  of  the  blood  vessels  of  the  ear, 
and  that  during  stimulation  of  the  nerve  with  an  electric  cur- 
rent, the  blood  vessels  become  very  small,  and  the  ear  conse- 
quently colder.  This  experiment  shows  that  the  nervous  system 
plays  an  important  role  in  the  control  of  the  flow  of  blood  through 
the  tissues,  and  from  it  many  important  truths  about  the  nervous 
control  of  the  blood  vessels  may  be  deduced.  If  cutting  a  nerve 
will  cause  the  blood  vessels  to  dilate,  and  stimulating  the  same 
nerve  with  an  electric  current  will  cause  the  vessels  to  constrict 
to  much  less  than  their  normal  size,  it  follows  that  the  blood 
vessels  must  be  normally  held  in  a  state  half  Way  between  ex- 
treme dilation  and  constriction  by  stimuli  received  from  the 
nervous  system.  The  nerve  fibers  which  carry  the  stimuli,  be- 
cause of  their  power  of  producing  constriction  of  the  blood  ves- 
sels, are  known  as  vasoconstrictor  nerve  fibers.    They  are  com- 


190  HUMAN   PHYSIOLOGY. 

parable  in  action  to  the  accelerator  nerves  to  the  heart,  since  stim- 
ulation of  either  type  of  nerve  tends  to  produce  an  increase  in  the 
blood  pressure,  the  one  by  quickening  the  heart  rate  and  the 
other  by  constricting  the  blood  vessels  and  increasing  the  resist- 
ance to  the  flow  of  blood. 

The  presence  of  the  vasoconstrictor  fibers  in  the  sympathetic 
nerves  is  easily  shown  by  the  fact  that  stimulation  of  these  nerves 
to  any  part  of  the  body  produces  a  marked  diminution  in  the 
size  of  the  part  to  which  the  nerves  are  connected.  At  the  same 
time  there  is  an  increase  in  the  general  blood  pressure,  because 
the  freedom  of  outflow  of  blood  from  the  arterial  system  is  some- 
what reduced.  The  large  nerves  which  supply  the  limbs  also 
contain  vasoconstrictor  nerves.  These  are  derived  from  fibers 
coming  from  the  ganglia  of  the  sympathetic  chain  in  the  thorax 
and  abdomen  and  joining  with  the  roots  of  the  spinal  nerves  in 
order  that  the  fibers  may  be  distributed  along  with  the  cerebro- 
spinal nerves  to  the  part  in  question  (see  p.  277). 

After  section  of  the  spinal  cord,  the  blood  vessels  of  the  part 
of  the  body  supplied  with  vasoconstrictor  nerves  below  the  level 
of  the  section  of  the  cord,  become  dilated,  and  may  be  constricted 
again  if  a  stimulus  be  applied  to  the  lower  end  of  the  cord.  The 
effect  of  such  a  stimulus  is  to  increase  the  blood  pressure,  since 
the  resistance  offered  to  the  flow  of  blood  is  increased. 

The  organ  in  which  the  changes  taking  place  in  the  blood 
vessels  under  various  conditions  can  be  most  easily  demonstrated 
is  the  kidney.  It  is  not, hard  to  enclose  one  kidney  in  an  air- 
tight box,  and  by  means  of  rubber  tubing  to  connect  the  box 
with  an  instrument  called  a  tamtour,  which  will  record  on  a 
smoked  drum  any  change  in  the  amount  of  air  in  the  box, 
i.  e.,  any  increase  in  kidney  volume  will  cause  air  to  pass  out 
of  the  box,  or  the  reverse  in  case  the  kidney  volume  decreases. 
The  instrument  is  called  a  plethysmo graph.  The  instrument 
applied  to  the  kidney  of  anaesthetized  animals  records  each  heart 
beat,  or,  in  other  words,  shows  a  pulse.  Any  change  in  blood 
pressure  will  also  cause  a  change  in  kidney  volume,  but  the 
nature  of  the  change  will  depend  on  the  cause  of  the  change 
of  blood  pressure.     (See  Fig,  26).  ' 


THE   VASOMOTOR   NERVES.  191 

The  vasomotor  nerves  to  the  kidney  and  the  abdominal  viscera 
are  for  the  most  part  supplied  by  the  lower  thoracic  nerves. 
These  sympathetic  fibers  are  combined  and  enter  tlie  abdomen  in 
what  are  known  as  the  splanchnic  nerves,  which  terminate  about 
nerve  cells  in  a  ganglion  behind  the  stomach,  which  is  called  the 
semilunar  ganglion  of  the  solar  plexus.  If  while  the  normal  vol- 
ume of  the  kidney  is  being  recorded,  the  splanchnic  nerve  of  the 
corresponding  side  of  the  body  is  cut,  the  kidney  will  show  in- 
crease in  volume,  due  to  the  loss  of  the  vasoconstrictor  nerve 
control  on  its  vessels.  On  the  other  hand,  stimulation  of  the  cut 
end  of  the  splanchnic  nerve  leading  towards  the  kidney,  will 
produce  a  great  decrease  in  the  kidney  volume,  and  a  marked 
increase  in  the  systemic  blood  pressure,  due  to  a  diminution  in 
the  volume  of  the  vessels  of  the  kidney  and  of  the  whole  splanch- 
nic area,  since  the  splanchnic  nerves  supply  not  only  the  kid- 
neys, but  the  whole  intestinal  tract  with  vasoconstrictor  fibers. 

Vasodilator  Nerves. — There  is  another  class  of  efferent  nerve 
fibers  to  the  arteries,  which  are  known  as  the  vasodilator  nerves 
When  stimulated  they  bring  about  a  dilatation  of  the  arte- 
rioles, and  allow  a  greater  amount  of  blood  to  pass  through  the 
vessels.  Vasodilator  nerve  fibers  are  found  in  all  the  spinal 
nerves,  and  they  run  to  the  blood  vessels  along  with  the  nerve 
trunks  supplying  the  various  organs.  Unlike  the  vasoconstrictor 
nerves,  they  do  not  seem  to  be  continually  exerting  an  influence 
or  tonic  action  on  the  blood  vessels.  Because  their  action 
is  hard  to  elicit,  not  so  much  is  known  of  their  normal 
functions  as  is  known  of  the  vasoconstrictor  nerves.  In  some 
nerves,  however,  they  predominate  and  their  action  is  easily 
seen.  Such  is  the  case  in  the  chorda  tympani,  a  nerve  coming 
from  the  seventh  cranial  nerve  and  supplying  the  submaxillary 
gland  with  fibers,  which  when  stimulated  bring  about  an  increase 
in  the  flow  of  saliva  and  marked  dilatation  of  the  blood  vessels 
of  the  gland  (see  p.  41).  The  arterioles  normally  may  be  sup- 
posed to  be  held  in  a  state  midway  between  dilatation  and  con- 
traction. Stimulation  of  the  vasodilator  nerves  probably  in- 
hibits the  tonic  action  of  the  vasoconstrictor  nerves  and  the  mus- 
cles of  the  vessels  are  extended  by  the  force  of  the  arterial  blood 
XH'essure. 


192  HUMAN   PHYSIOLOGY. 

After  section  of  the  sciatic  nerve,  the  constrictor  fibers  soon 
die,  and  the  dilator  fibers,  which  live  for  a  time,  may  be  shown 
to  be  present  by  the  fact  that  the  volume  of  the  leg  increases 
when  the  nerve  is  stimulated. 

Vasomotor  Reflexes. — In  the  same  manner  that  the  heart  is 
influenced  by  afferent  stimuli  reaching  cardiac  centers  from  peri- 
pheral parts  of  the  body,  we  find  afferent  stimuli  affecting  the 
size  of  the  blood  vessels  reflexly  by  way  of  the  vasomotor  center 
— located  in  the  medulla  near  the  vagus  center — and  the  vaso- 
motor nerves.  Some  of  the  afferent  impulses  cause  dilation  of 
the  blood  vessels,  while  others  cause  constriction.  Perhaps  the 
most  important  of  the  sensory  nerves,  which,  when  stimulated, 
produce  a  dilation  of  the  blood  vessels,  is  the  cardiac  depressor, 
which  we  mentioned  in  connection  with  the  afferent  nerves  of 
the  heart.  It  will  be  remembered  that  this  nerve  has  sensory 
endings  in  the  left  ventricle  and  in  the  aorta,  and  that  these  are 
stimulated  when  the  blood  pressure  in  the  arterial  system  reaches 
too  great  a  height  for  the  safety  of  the  individual.  The  stimuli 
originating  in  the  sensory  endings  of  this  nerve  are  carried  to 
the  cardiac  center  and  are  then  transmitted  to  the  heart  through 
the  vagus  nerves.  Besides  the  slowing  of  the  heart  which  is  thus 
produced,  there  also  occurs  a  dilation  of  the  peripheral  vessels 
brought  about  by  the  action  of  the  stimuli  on  the  vasomotor 
center.  This  is  easily  demonstrated  by  electrically  stimulating 
the  cardiac  depressor  nerve  after  both  vagi  have  been 
cut  in  the  neck  and  the  reflex  vagus  action  thus  removed.  The 
fall  of  blood  pressure  which  is  obtained  under  these  conditions 
is  due  to  an  inhibition  of  the  constrictor  center  and  a  stimula- 
tion of  the  dilator  center  of  the  vasomotor  nerves. 

The  stimulation  of  many  of  the  afferent  or  sensory  nerves  of 
the  body  is  followed  by  a  change  in  the  blood  pressure.  Just 
what  this  change  may  be  it  is,  often  impossible  to  predict.  Strong 
sensory  stimuli  of  short  duration  may  produce  a  marked  rise  in 
blood  pressure,  the  constrictor  center  being  the  most  affected. 
On  the  other  hand,  if  the  stimuli  are  very  strong  or  continued 
over  a  long  period  of  time,  the  constrictor  nerves  may  become 
exhausted,  as  it  were,  resulting  in  a  dilation  of  the  arteries  and 


THE   VASOMOTOR   REFLEXES.  193 

a  fall  in  the  general  blood  pressure.  Like  phenomena  are  often 
seen  following  fright,  pain,  grief,  and  excitement.  The  patient 
becomes  suddenly  pale,  dizzy,  and  may  faint,  losing  conscious- 
ness entirely.  This  is  due  to  a  fall  in  the  arterial  blood  pressure 
produced  by  a  temporary  inhibition  of  the  vasoconstrictor  nerves 
and  perhaps  also  by  a  slowing  of  the  heart,  due  to  vagus  stimula- 
tion. If  the  person  be  standing,  the  blood  naturally  flows  to  the 
vessels  of  the  abdominal  viscera  and  dependent  portions  of  the 
body,  and  the  brain  is  thereby  rendered  bloodless.  The  treat- 
ment of  these  cases  is  to  elevate  the  feet  and  abdomen  and  to 
lower  the  head. 

In  case  the  depression  of  the  blood  pressure  slowly  develops 
because  of  the  gradual  onset  of  fatigue  in  the  vasomotor  and 
other  nervous  centers,  a  condition  known  as  surgical  shock  super- 
venes. The  treatment  of  this  condition  demands  plenty  of  air, 
stimulants,  saline  or  blood  transfusion,  and  measures  to  main- 
tain the  body  temperature. 

The  Pressure  Effects  of  Gravity  on  the  Blood  Flow  vary 
according  to  the  posture  of  the  body.  In  the  upright  position 
the  blood  vessels  of  the  feet  support  a  column  of  blood  of  rela- 
tively great  height,  but  when  the  individual  is  lying  down  this 
ceases  to  be  the  case.  In  spite  of  this,  by  means  of  the  delicate 
adjustments  which  the  nervous  system  can  bring  about  in  the 
heart  and  the  blood  vessels,  there  is  little  difference  in  the  pres- 
sure of  the  blood  in  the  arteries  in  any  position  which  the  person 
may  assume.  The  blood  vessels  and  nerves  soon  lose  this  power 
if  it  is  not  continuously  exercised.  This  is  illustrated  in  patients 
who  have  been  confined  to  their  beds  for  a  time.  If  they  try  to 
walk  or  to  stand  up  suddenly,  they  become  very  dizzy  and  may 
faint,  which  means  that  the  blood  has  left  the  vessels  of  the 
brain  and  is  gathered  by  the  force  of  gravity  in  the  vessels  of 
the  dependent  parts  of  the  body.  With  a  normal  vasomotor 
mechanism,  the  vessels  of  the  feet  and  viscera  would  quickly 
constrict  to  such  an  Extent  that  the  blood  pressure  would  remain 
at  its  normal  height  in  the  vessels  of  the  brain. 

The  fact  that  stimulation  of  sensory  nerves  by  the  gross  meth- 
ods of  the  laboratory  results  in  very  profound  changes  in  the 


194  HUMAN   PHYSIOLOGY. 

blood  pressure  and  in  the  velocity  of  the  circulation  of  the  bloody 
suggests  that  normally  the  vasomotor  and  cardiac  nerves  play 
an  important  role  in  the  proper  distribution  of  blood  in  the 
various  parts  of  the  body.  It  may  be  supposed  that  normally  the 
nerves  of  the  vascular  system  function  to  control  the  blood  flow 
through  the  various  organs  according  to  their  respective  needs. 
Whenever  the  work  of  an  organ  is  increased,  the  blood  flow  like- 
wise is  augmented  in  the  part,  while  in  the  rest  of  the  body  the 
blood  flow  is  diminished  to  a  greater  or  less  extent.  The  blood 
supply  is  continually  changing  according  to  the  call  of  the  vari- 
ous tissues  for  blood ;  now  the  muscles,  now  the  digestive  organs, 
now  the  brain 'demand  more  blood,  and  this  is  supplied  in  the 
proper  amount  by  the  nervous  system  commanding  some  arte- 
rioles to  dilate  and  others  to  constrict. 

Haemorrhage. — The  action  of  the  vasomotor  mechanism  is 
beautifully  shown  in  the  case  of  haemorrhage.  As  blood  is  with- 
drawn, the  vasomotor  nerves  are  stimulated  by  the  falling  pres- 
sure in  the  brain.  This  brings  about  a  more  powerful  tonic  con- 
striction of  the  vessels  through  the  action  of  vasoconstrictor 
nerves,  the  vascular  area  becomes  smaller  and  smaller  in  size, 
and  less  blood  is  required  to  maintain  the  blood  pressure.  Be- 
cause of  this  mechanism  a  relatively  large  amount  of  blood  can 
be  lost  without  affecting  the  general  blood  pressure  (Fig.  26, 
No.  V)". 

The  Regulation  of  the  Blood  Supply  by  Chemical  Stimuli. — 
The  caliber  of  the  blood  vessels  may  be  influenced  by  other  means 
than  through  their  nervous  mechanism.  Acids  in  very  small 
concentrations  cause  a  vascular  dilatation.  For  example,  lactic 
acid  and  carbonic  acid,  both  of  which  are  formed  during  muscu- 
lar work,  may  produce  a  local  dilatation  of  the  blood  vessels,  the 
phenomenon  thus  constituting  an"  automatic  mechanism  for  deliv- 
ering more  blood  to  a  part  when  it  is  needed.  On  the  other  hand, 
the  secretion  of  the  adrenal  and  of  a  portion  of  the  pituitary 
gland  (see  p.  131)  produces  a  constriction  of  the  vessels  and 
thus  tends  to  maintain  the  normal  blood  pressure.  Recently  it 
has  been  shown  that  during  periods  of  excitement  and  sensory 
pain  the  amount  of  the  adrenal  secretions  may  be  increased  and 


ASPHYXIA.  195 

the  arterial  blood  pressure  raised  as  a  result  of  general  vasocon- 
striction (p.  190).  Because  of  its  vasoconstricting  properties, 
extract  of  the  adrenal  glands  (''adrenalin"  or  " epinephrin") 
is  used  in  local  anesthetics,  as  in  cocain  solution,  to  prevent 
bleeding  and  to  minimize  the  absorption  of  the  coeain  into  the 
general  circulation  (Fig.  26,  No.  IV). 

Asphyxia. — Whenever  the  amount  of  oxygen  which  the  blood 
must  supply  to  the  tissues  falls  below  the  minimum  amount  re- 
quired, a  condition  known  as  asphyxia  develops.  If  the  nervous 
centers  are  intact,  any  interference  with  the  respiratory  function, 
as  by  obstruction  of  the  respiratory  passages,  lack  of  ogygen  in 
the  atmosphere,  or  the  presence  of  irrespirable  gases  in  the  at- 
mosphere— such  as  carbon  monoxide,  which  reduces  the  oxygen 
capacity  of  the  haemoglobin — interferes  with  the  blood  supply  of 
the  brain  and  will  produce  a  train  of  phenomena  in  which  the 
respiratory  and  circulatory  changes  are  prominent.  In  ordinary 
asphyxia  two  factors  may  be  involved,  a  deficiency  of  oxygen 
and  an  excess  of  carbon  dioxide  in  the  blood.  The  phenomena 
following  each  are  essentially  the  same,  and  may  be  divided  into 
three  typical  stages.  In  the  first  stage,  that  of  hyperpnoea,  the 
respirations  are  increased  in  rate  and  amplitude.  This  stage 
merges  into  the  second,  which  consists  of  exaggerated  expiratory 
efforts,  and  loss  of  consciousness ;  stimulation  of  the  vascular  cen- 
ters in  the  brain,  causing  general  vasoconstriction  accompanied 
with  vagus  slowing  of  the  heart  also  occurs.  The  net  result  is  a 
rise  in  blood  pressure.  In  the  third  stage,  the  expiratory  efforts 
give  way  to  slow  deep  inspirations  followed  by  expiratory  con- 
vulsions. The  pupils  dilate  widely,  the  heart  becomes  very  weak 
from  lack  of  oxygen  and  overwork,  and  death  occurs  from  car- 
diac failure.  The  changes  produced  in  the  respiratory  move- 
ments, as  well  as  those  of  the  vascular  system,  are  caused  by  the 
direct  stimulation  of  the  respiratory  (see  p.  220)  and  vascular 
centers,  by  excess  of  carbon  dioxide  and  by  the  lack  of  oxygen 
in  the  blood. 

Nitrous  Oxide. — The  circulatory  and  respiratory  changes  ac- 
comxmnying  tlie  administration  of  nitrous  oxide  gas  are  very 
similar  to  those  produced  in  asphyxia.     The  asphyxia  produced 


196  HUMAN  PHYSIOLOGY. 

by  the  lack  of  oxygen  and  the  excess  of  carbon  dioxide  in  the 
blood  during  gas  anesthesia,  stimulates  the  vasoconstrictor  cen- 
ter, producing  a  rise  in  blood  pressure.  The  narcotic  action  of 
the  gas  depresses  the  inhibitory  effects  of  the  vagus  cardiac  cen- 
ter on  the  heart.  The  heart  is  therefore  quickened  and  tends 
still  further  to  increase  the  blood  pressure.  For  these  reasons  it 
is  not  wise  to  use  nitrous  oxide  in  the  case  of  elderly  patients 
with  weakened  sclerosed  arteries,  or  in  the  case  of  those  suffering 
from  cardiac  disease.  When  oxygen  is  given  along  with  the 
nitrous  oxide  the  asphyxial  phenomena  are  reduced. 

Cocain. — The  effect  of  cocain  injections  on  the  circulation  are 
both  central  and  peripheral,  and  vary  according  to  the  dose  and 
the  individual  susceptibility.  Very  small  doses  generally  cause 
a  slight  fall  in  blood  pressure,  due  to  slowing  of  the  heart  from 
stimulation  of  the  vagus.  The  vasomotor  center  is  likewise  stim- 
ulated, but  the  resulting  vasoconstriction  does  not  compensate 
for  the  fall  in  pressure  caused  by  the  decreased  action  of  the 
heart.  Moderate  doses  depress  the  vagus  function  and  increase 
the  heart  rate,  which,  together  with  the  vasoconstrictor  stimula- 
tion observed  in  the  case  of  the  smaller  doses,  causes  a  marked 
rise  in  the  blood  pressure.  Large  doses  paralyze  the  vital  centers 
in  the  medulla,  and  a  great  fall  in  blood  pressure  results.  "With 
small  doses*  the  respirations  are  accelerated,  but  in  fatal  doses 
the  respiratory  center  (see  p.  219)  is  paralyzed  and  death  ensues. 


CHAPTER  XXI. 
THE   RESPIRATION. 

Oxygen  is  one  of  the  essential  substances  required  by  every 
living  organism,  in  the  cells  of  which  it  combines  with  the  carbon 
to  form  carbon  dioxide,  and  with  hydrogen  to  form  water.  All 
the  phenomena  accompanying  the  supply  and  utilization  of  oxy- 
gen and  the  excretion  of  carbon  dioxide  are  included  under  the 
subject  of  respiration. 

In  the  simplest  ^forms  of  life  the  exchange  of  oxj^gen  and  car- 
bon dioxide  gas  occurs  directly  with  the  air,  but  in  more  complex 
organisms  this  sort  of  exchange  is  impossible,  since  practically 
none  of  the  cells  composing  the  organism  is  in  direct  communi- 
cation with  the  air.  Some  sort  of  respiratory  apparatus  becomes 
necessary,  so  that  each  cell  may  be  supplied  with  oxygen  and 
have  its  carbon  dioxide  removed.  In  the  higher  animals  this  is 
accomplished  through  the  agency  of  the  blood,  which  is  well 
adapted  to  transport  the  oxygen  and  carbon  dioxide,  first 
because  it  contains  chemical  bodies  with  which  the  gases  can 
unite,  and  secondly  because  it  comes  in  close  contact  with  the 
tissue  cells  in  the  peripheral  portions  of  the  body,  and  with  the 
atmospheric  air  in  the  capillaries  of  the  lungs.  The  study  of 
the  respiratory  function  therefore  includes  the  mechanism  of 
the  gas  exchange  between  the  tissues  and  the  blood,  or  internal 
respiration,  and  also  that  between  the  lungs  and  the  blood,  or 
external  respiration. 

Internal  Respiration.  • 

The  energy  which  the  body  expends  in  the  performance  of  the 
functions  of  life,  including  the  heat  which  is  required  to  main- 
tain the  body  temperature,  is  produced  in  the  cellular-  chemical 
reactions,  in  which  the  oxygen  of  the  air  combines  with  the 
hydrogen  and  carbon  of  the  foodstuffs  to  form  water  and  carbon, 
dioxide  gas. 

197 


198  HUMAN   PHYSIOLOGY. 

Oxidation  in  the  Tissues. — The  actual  mechanism  which 
unites  the  oxygen  with  the  carbon  and  hydrogen,  of  the  food- 
stuffs within  the  tissue  cells,  is  not  entirely  known.  In  spite  of 
the  fact  that  the  processes  of  combustion  of  hydrocarbon  matter 
outside  the  body  yield  the  same  end  products  as  the  oxidations 
taking  place  within  it,  the  two  processes  are  not  strictly  analo- 
gous. An  important  point  of  difference  lies  in  the  fact  that  the 
intracellular  materials^fats,  proteins,  and  carbohydrates — are 
oxidized  with  relatively  great  rapidity  at  low  temperatures 
(98.4°),  whereas  the  same  reactions  outside  the  body  require  a 
very  high  temperature. 

Let  us  take  as  an  example  the  cell  of  the  yeast  plant,  in  which 
there  is  a  substance,  under  the  influence  of  which,  the  sugar 
molecule  becomes  split  up,  at  a  temperature  below  that  of  the 
body,  to  produce  carbon  dioxide  and  water.  Similar  substances 
are  present  in  the  tissue  cells  of  plants  and  animals ;  they  are ' 
the  ferments  or  enzymes  (see  p.  34),  and  they  act  as  catalytic 
agents.  The  function  of  these  bodies  is  to  increase  the  velocity 
of  many  chemical  reactions  which  otherwise  proceed  so  slowly 
that  they  may  be  said  in  some  cases  not  to  exist.  A  class  of 
these  substances  is  present  within  the  tissue  cells,  which  at  the 
demand  of  the  tissues  control  the  extent  and  the  velocity  of  the 
union  of  oxygen  with  the  hydrocarbons  of  the  food.  Such  en- 
zymes are  known  as  oxidases. 

"What  evidence  have  we,  however,  that  this  oxidation  takes 
place  within  the  tissues  and  not  within  the  blood  itself?  It  is 
conceivable  that  the  substances  that  are  to  be  oxidized  are  col- 
lected from  the  tissues  by  the  blood,  and  that  the  oxygen  combines 
with  them  in  this  fluid.  It  is  quite  possible  that  some  oxidation 
takes  place  in  the  blood,  for  it  is  essentially  a  tissue  and  has 
a  metabolism  of  its  own,  but  this  is  not  true  for  the  oxidation 
which  concerns  the  tissues,  since  this  takes  place  in  the  tissues 
themselves,  as  can  be  shown  by  the  following  fact :  The  blood  of 
a  frog  may  be  replaced  with  saline  solution  in  which  oxygen  is 
dissolved  under  pressure,  without  killing  the  animal.  It  is 
hardly  conceivable  that  oxidation  similar  to  that  occurring  with- 
in the  body  can  take  place  in  a  solution  of  sodium  chloride. 


OXIDATION    IN    THE   TISSUES.  199 

Relation  of  Oxidative  Process  to  Activity. — Under  ordinary 
conditions  the  blood  has  a  supply  of  food  and  oxygen  sufficient 
for  the  needs  of  the  body.  An  excess  of  either  does  not  intensify 
the  oxidative  process.  An  animal  will  give  off  the  same  amount 
of  carbon  dioxide  in  an  atmosphere  of  pure  oxygen  as  it  will 
under  ordinary  conditions.  Tliis  fact  indicates  that  the  oxida- 
tive processes  are  governed  not  by  the  supply  of  food  or  oxygen, 
but  rather  by  the  actual  needs  of  the  tissues.  A  muscle  freshly 
removed  from  the  body  may  be  made  to  contract,  and  will  give 
off  carbon  dioxide  for  some  time  in  the  entire  absence  of  oxygen 
in  the  surrounding  medium.  Another  feature  of  this  experiment 
is  that  for  a  time  after  the  muscle  has  ceased  to  contract,  it  will 
produce  heat  and  take  up  a  large  amount  of  oxygen.  Indeed 
the  maximal  intake  of  oxygen  and  output  of  heat  often  occurs 
after  the  actual  period  of  work.  In  this  respect  the  muscle  can 
be  likened  to  a  storage  battery  which  is  charged  by  the  actual 
expenditure  of  energy  and  delivers  quickly  the  energy  stored 
up  when  the  circuit  is  closed.  If  the  volume  intake  of  oxygen 
and  output  of  carbon  dioxide  is  measured,  it  will  be  found  that 
the  amounts  are  greatly  increased  during  periods  of  tissue  activ- 
ity. Pjxperiments  liave  demonstrated  that  a  muscle  at  full  work 
will  use  up  its  own  volume  of  oxygen  in  ten  miputes.  To  supply 
such  an  amount  of  oxygen  requires  a  very  high  degree  of  effi- 
ciency on  the  i)ai't  of  the  distributing  agent,  the  blood. 

Physical  Laws  Governing  Solution  op  Gases. — A  brief  re- 
view of  the  physical  laws  governing  the  solution  of  gases  in  water 
will  help  us  materially  to  understand  the  mechanism  of  the  trans- 
portation of  oxygen  and  carbon  dioxide  by  the  blood  and  the 
respiratory  mechanism  in  general. 

Gases  differ  from  solid  and  fluid  materials  in  that  the  parti- 
cles which  compose  them  repel  more  than  they  attract  each 
other,  thus  permitting  the  gas  to  diffuse  throughout  the  atmos- 
pliere.  The  r'-pclling  force  exerted  by  the  molecules  of  gas  on 
th(!  walls  of  the  container  produce  the  phenomena  of  gaseous 
pressure.  It  follows,  therefore,  that  the  pressure  which  a  gas 
exerts  varies  with  the  number  of  molecules  of  the  gas  present 
in  the  atmosphere.     The  various  gases  diffuse  out  into  space 


200  HUMAN   PHYSIOLOGY. 

until  this  diffusion  pressure  is  balanced  by  tbe  force  of  gravity. 
The  weight  of  a  substance  is  the  force  which  gravity  exerts  on 
it.  The  weight  of  a  gas  would  therefore  be  an  indication  of  the 
diffusion  pressure  of  the  gas  in  the  atmosphere,  and  also  indi- 
rectly of  the  number  of  molecules  of  gas  present.  The  weight 
of  a  gas  or  the  pressure  of  the  gas,  on  the  earth's  surface  is 
measured  by  an  instrument  called  a  barometer,  in  which  the 
atmosphere  is  balanced  against  a  long  column  of  mercury,  and 
the  weight  expressed  in  the  number  of  millimeters  of  mercury 
which  the  atmosphere  will  support.  At  sea  level  and  at  15.5° 
Centigrade,  the  pressure  which  the  atmosphere  exerts  on  the 
surface  of  any  fluid  is  sufficient  to  support  the  weight  of  a  col- 
umn of  mercury  760  millimeters  in  weight. 

The  solubility  of  a  gas  in  a  fluid  is  measured  by  the  number  of 
cubic  centimeters  of  gas  which  one  cubic  centimeter  of  fluid  will 
dissolve  under  standard  conditions  of  temperature  and  pressure. 
Such  a  figure  is  known  as  the  coefficient  of  solubility.  For  ex- 
ample, pure  carbon  dioxide  gas  under  standard  conditions  of 
temperature  and  pressure  (760  mm.  pressure  "and  15.5  degrees 
Cent.)  will  dissolve  to  the  amount  of  one  c.  c.  in  one  c.  c.  of  water. 
Under  like  conditions  only  0.04  c.  c.  of  oxygen  will  be  dissolved. 
The  coefficient  of  solubility  of  carbon  dioxide  is  therefore  1.0  and 
of  oxygen  0.04. 

The  amount  of  gas  which  will  go  into  solution  in  water  depends 
on  three  factors :  the  temperature  of  the  water,  the  solubility  of 
the  gas  in  water,  and  the  pressure  which  the  gas  exerts  on  the 
surface  of  the  water.  As  a  rule,  the  higher  the  temperature  of  the 
water,  the  less  gas  will  go  into  solution,  or  in  other  words,  the 
solubility  of  a  gas  varies  inversely  with  the  temperature. 

If,  in  place  of  having  pure  gases  over  a  fluid,  a  mixture  of 
several  gases  be  present,  then  we  find  the  solubility  of  each  of 
the  gases  varying  directly  with  the  pressure  it  exerts  on  the  sur- 
face of  the  fluid.  Suppose  that  in  place  of  exposing  a  cubic  centi- 
meter of  water  to  oxygen  at  760  mm.  pressure,  we  expose  it  to 
oxygen  at  a  pressure  of  152  mm.  mercury,  which  is  the  normal 
pressure  of  -  oxygen  in  the  air  (1/5  of  an  atmosphere)  it  would 
absorb  1/5  of  .04  c.  e.  or  .008  c.  c.  of  oxygen.    The  presence  of 


HEMOGLOBIN.  201 

other  gases  does  not  enter  into  consideration,  for  according  to 
Dalton-Henry's  law,  when  two  or  more  gases  are  mixed  together, 
each  of  them  produces  the  same,  pressure  as  if  it  separately  occu- 
pied the  entire  space  and  the  other  gases  were  absent.  When  the 
fluid  has  taken  up  all  the  gas  it  can,  an  equilibrium  becomes  es- 
tablished between  the  gas  in  the  atmosphere  and  the  gas  within 
the  fluid.  The  pressure  which  the  gas  in  the  fluid  exerts  on  the 
gas  in  the  atmosphere  is  known  as  the  tension  of  the  gas,  and 
equals  the  pressure  of  the  gas  in  the  outside  atmosphere  to  which 
it  is  exposed.    This  can  be  easily  measured. 

Since  the  pressure  of  the  oxygen  in  the  air  in  the  lungs  is  less 
than  that  in  the  outside  atmosphere,  it  is  apparent  that  if  the 
blood  should  carry  the  same  amount  of  oxygen  as  water  does,  the 
amount  would  be  very  small  indeed.  Analysis  of  the  amount  of 
oxygen  in  arterial  blood  shows  that  it  contains  40  times  the 
amount  per  c.  c.  that  water  can  dissolve  under  like  conditions. 
For  example,  let  us  imagine  human  blood  to  be  water.  It  would 
carry  then  only  1/40  of  the  volume  of  oxygen  that  it  does,  and 
the  tissues  of  the  body  would  need  a  vascular  system  the  size  of 
an  elephant's  in  order  to  obtain  as  much  oxygen  as  normally 
is  supplied  by  the  blood.  Therefore  it  is  obvious  that  the  laws 
for  simple  solutions  can  apply  only  in  a  slight  degree  to  the 
gases  in  the  blood.  They  would  account  at  the  most  for  only  0.7 
per  cent  of  the  total  oxygen  and  2  per  cent  of  the  carbon  dioxide 
found  in  the  blood. 

Haemoglobin. — The  extraordinary  ability  of  the  blood  to 
carry  oxygen  and  carbon  dioxide  lies  in  the  presence  of  sub- 
stances capable  of  chemically  uniting  with  and  storing  up  large 
amounts  of  the  gases.  The  iron — containing  protein  substance 
called  haemoglobin,  found  in  the  red  blood  cells,  carries  the 
oxygen,  and  the  alkalies  and  proteins  of  the  blood  carry  most  of 
the  carbon  dioxide.  Analysis  of  samples  of  arterial  and  venous 
blood  gives  the  following  average  figures,  which  represent  the 
volumes  of  the  gas  found  in  one  hundred  volumes  of  blood. 

Oxygen  CO  Nitrogen 

100  vol.  arterial  blood  contains 20  40  1-2 

100  vol.  venous  blood  contains 10-12  45-50  1-2 


202  HUMAN   PHYSIOLOGY. 

The  small  amount  of  nitrogen  present  in  the  blood  in  spite  of 
the  large  percentage  found  in  the  atmosphere  (4/5  of  the  baro- 
metric pressure  being  due  to  nitrogen)  is  due  to  the  absence  of 
any  chemical  body  within  the  blood  plasma  which  will  unite  with 
nitrogen.  Of  the  20  volumes  per  cent  of  oxygen  found  in  arterial 
blood  only  0.7  per  cent  is  in  solution  in  the  plasma. 

Relation  of  Oxygen  to  Hemoglobin. — In  order  to  under- 
stand the  affinity  of  oxygen  for  hgemoglobin,  we  must  investigate 
the  various  conditions  which  favor  the  union  of  hemoglobin  with 
oxygen  and  the  break-down  of  the  resulting  oxygen  compound, 
oxyhaemoglobin,  into  oxygen  and  heemoglobin.  Equal  quantities 
of  pure  Jiceniogloiin  solution  are  placed  in  a  series  of  glass  ves- 
sels containing  variable  quantities  of  oxygen  mixed  with  nitrogen 
at  atmospheric  pressure.  After  shaking,  the  solutions  are  removed 
and  the  amount  of  oxygen  in  each  sample  is  measured. 

The  hgemoglobin  solutions  in  the  tubes  containing  a  partial 
pressure  of  oxygen  which  is  within  two-thirds  of  that  present  in 
air  (between  90  and  152  mm.  of  mercury)  are  all  almost  satu- 
rated with  oxygen.  In  other  words,  at  these  pressures  the  hsemo- 
globin  exists  entirely  in  the  form  of  oxyhaemoglobin.  In  the  tube 
containing  one-half  the  pressure  of  oxygen  in  air  (i.  e.,  almost  76 
mm.  Hg. ) ,  the  hgemoglobin  solution  is  90  per  cent  saturated.  At 
about  one-fourth  the  normal  oxygen  pressure  in  air  (i.  e.,  40  mm. 
Hg.),  it  is  about  84  per  cent  saturated.  At  lower  partial  pres- 
sures of  oxygen,  the  ability  of  hgemoglobin  to  unite  with  oxygen 
very  rapidly  decreases. 

From  these  observations  we  must  conclude  that,  as  the  pres- 
sure of  oxygen  in  contact  with  the  hgemoglobin  solution  increases 
above  zero,  by  graded  stages,  the  amount  of  oxygen,  per  unit  of 
increase  of  oxygen  pressure,  that  combines  with  hsemoglobin  fit 
low  pressures  is  large,  but  becomes  relatively  less  at  higher  pres- 
sures. Or,  conversely,  if  the  haemoglobin  saturated  with  oxygen 
be  subjected  to  decreasing  oxygen  i^ressure,  the  combined  oxygen 
is  set  free  at  first  slowly  and  then  more  rapidly. 

If  the  oxygen-combining'  power  of  Mood  be  investigated  in  ex- 
actly the  same  way  as  described  above  and,  the  results  compared 
with  those  of  a  pure  haemoglobin  solution,  a  marked  difference 


HEMOGLOBIN. 


203 


will  be  observed.  At  low  pressures  the  oxygen  is  more  easily  re- 
leased from  the  haemoglobin  of  the  blood  than  from  pure  solu- 
tions of  hfemoglobin.  An  inquiry  into  the  cause  of  this  difference 
has  revealed  the  following  facts.  The  rate  at  which  oxyhaemo- 
globin  breaks  down  into  oxygen  and  haemoglobin,  depends  on 

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27 


Ordinate.s — Percentage   .saturation   of   haemoglobin    with   oxygen. 

Abscissae — Tension  of  oxygen  in  mm.  of  meicury. 

Curve    A — Degree    of    saturation    of    pure    htemoglobin    .solutions    at    varying 

pressures. 
Curve   B — Modification   of   degree  of   saturation   caused   by   presence  of  salts 

in   the  blood. 
Curve  C — Effect  of  20  mm.  CO^  pressure  on  above  solution. 

Cui-ve    IJ — The   saturation   curve   in    normal    blood   at    40   mm.    carbon    dioxide 
pressure. 


204  HUMAN  PHYSIOLOGY. 

other  factors  besides  oxygen  pressure.  These  are:  (1)  tempera- 
ture, (2)  the  presence  of  inorganic  salts,  and  (3)  carbon  dioxide 
or  other  weak  acids  in  the  blood.  If  haemoglobin  be  dissolved  in 
a  saline  solution  containing  the  same  concentration  of  inorganic 
salts  as  is  found  in  blood,  it  will  take  up  oxygen  in  a  manner 
somewhat  similar  to  blood  under  like  oxygen  pressures.  The 
similarity  will  become  perfect  if  the  saline  solutions  of  hemo- 
globin be  subjected  to  the  same  pressure  of  carbon  dioxide  as  that 
present  in  the  sample  of  blood,  that  is,  provided  the  temperature 
is  the  same  in  the  two  cases.  Of  the  curves  shown  in  Fig.  27,  a 
represents  the  degree  of  dissociation  of  oxygen  from  pure  oxy- 
hasmoglobin  solution  at  varying  oxygen  pressures ;  h,  the  modifi- 
cation in  the  degree  of  the  association  produced  by  the  presence 
of  the  salts  of  the  blood;  c,  the  effect  of  carbon  dioxide  on  the 
oxygen  content  of  the  haemoglobin  in  a  saline  solution;  and  d  is 
the  dissociation  curve  of  normal  blood  with  a  carbon  dioxide 
tension  of  40  mm. 

The  effect  of  carbon  dioxide  is  of  special  interest.  It  is  seen 
that  the  greater  the  concentration  of  carbon  dioxide,  the  more 
readily  is  the  oxygen  dissociated  from  the  oxyhaemoglobin.  Thus, 
at  an  oxygen  pressure  of  20  mm.  of  mercury,  the  amount  of 
oxyhsemoglobin  formed  is  67.5  per  cent  of  the  total  haemoglobin 
at  a  carbon  dioxide  pressure  of  5  mm.,  whereas  at  a  pressure  of 
40  mm.  of  carbon  dioxide  the  amount  of  oxyhaemoglobin  is  only 
29.5  per  cent.  Inasmuch  as  the  amount  of  carbon  dioxide  is  con- 
stantly changing  in  arterial  and  venous  blood,  the  presence  of 
this  gas  would  seem  to  be  an  important  factor  in  the  control  of 
the  oxidation  or  the  dissociation  of  the  hsemoglobin  compounds. 
At  any  rate,  it  would  help  to  account  for  the  ease  with  which 
oxygen  is  broken  from  the  oxyhaemoglobin  molecule  in  the  capil- 
laries which  are  imbedded  in  the  tissues  where  the  carbon  dioxide 
is  formed,  and  its  pressure  is  correspondingly  high. 

The  Mechanism  of  the  Respiratory  Exchange. — The  oxygen 
in  the  alveoli  or  air  passages  of  the  lungs  comprises  about  14  to 
15  per  cent  of  the  total  air,  and  exerts  on  the  cells  of  the  respira- 
tory epithelium  a  pressure  of  about  100  mm.  mercury,  more  or 
less.    Venous  blood  when  it  reaches  the  lungs  contains  about  50 


RESPIKATOBY  EXCHANGE.  205 

per  cent  less  oxj^gen  than  does  arterial  blood,  and  can  take  up 
from  6  to  8  c.  c.  of  oxygen  for  every  one  hundred  e.  c.  of  blood. 
Haemoglobin  solutions  are  almost  completely  saturated  with  oxy- 
gen at  pressures  of  oxygen  much  less  than  100  mm.  of  mercury. 
There  are,  therefore,  very  favorable  conditions  in  the  lungs  for 
haemoglobin  to  take  up  oxygen  from  the  air.  It  must  be  under- 
stood, however,  that  the  haemoglobin  does  not  obtain  oxygen  di- 
rectly from  the  air.  The  haemoglobin  is  held  in  the  blood  corpus- 
cles which  are  floating  in  the  blood  plasma.  Between  the  plasma 
and  the.  air  in  the  lungs  lie  two  thin  membranes,  the  capillary  wall 
and  the  wall  lining  the  air  sac  of  the  lung.  The  oxygen  must  first 
be  dissolved  by  the  fluid  in  the  lung  epithelium;  from  this  the 
cells  of  the  capillary  walls  take  oxygen,  and  the  plasma  in  turn 
takes  the  oxygen  from  the  capillary  cells.  The  plasma  loses  the 
oxygen  thus  obtained  because  the  hsemoglobin  is  very  greedy  for 
oxygen.  There  is  accordingly  a  difference  in  the  oxygen  pressure 
in  the  plasma  of  the  capillaries  of  the  lungs  sufficient  to  account 
for  the  absorption  of  oxygen  by  the  haemoglobin  of  the  blood.  The 
blood  leaving  the  lungs  is  delivered  into  the  left  heart,  from 
which  it  is  distributed  over  the  body.  Since  oxidation  takes 
place  within  the  tissue  cells,  oxygen  is  being  continually  called 
for,  and  the  lymph  surrounding  the  cells  must  continually  gain  a 
fresh  supply  of  oxygen  from  the  plasma  of  the  blood.  This  re- 
duces the  tension  of  oxygen  in  the  plasma  and  causes  an  evolution 
of  oxygen  from  the  dxyhsemoglobin,  which  is  taken  up  by  the 
plasma  to  be  passed  on  to  the  lymph  and  then  on  to  the  cell. 
There  is  thus  a  descending  scale  of  pressure  or  tension  of  oxygen 
from  the  air  of  the  lungs,  where  its  pressure  may  amount  to  100 
mm.  of  mercury,  until  it  reaches  the  tissue  elements,  where  the 
pressure  may  be  considered  zero.  Under  ordinary  conditions  the 
circulation  is  fast  enough  to  prohibit  the  complete  reduction  of 
the  oxyhaemoglobin.  In  case  it  is  not,  or  in  case  the  oxygen  sup- 
ply is  short,  the  phenomena  of  asphyxia  develop  (see  p.  195). 

Effect  of  Carbon  Dioxide  on  Oxyh.<emoglobin. — As  a  result 
of  the  oxidative  changes  which  take  place  within  the  cells,  carbon 
dioxide  is  produced,  and  the  tension  of  this  gas  rises  in  the  tis- 
sues.    It  will  be  remembered  in  the  discussion  of  the  dissociation 


206  HUMAN  PHYSIOLOGY. 

of  oxyhaemoglobin,  that  the  effect  of  increased  tensions  of  carbon 
dioxide  is  to  increase  the  rate  of  reduction  of  oxyhsemoglobin  into 
oxygen  and  haemoglobin.  Since  there  is  a  high  tension  of  car- 
bon dioxide  present  in  the  tissues  and  at  the  site  of  the  capil- 
laries, the  effect  on  the  reduction  of  oxyhgemoglobin  is  very 
marked,  and  has  a  great  influence  on  the  rate  at  which  oxygen  is 
supplied  to  the  tissues.  Just  as  there  is  a  descending  pressure 
of  oxygen  from  the  air  in  the  lungs  to  the  cell,  so  is  there  a  de- 
crease in  pressure  from  the  carbon  dioxide  in  the  cells  to  the  air 
of  the  lungs.  This  gas  therefore  passes  through  the  lymph  to  the 
plasma  and  out  of  the  plasma  through  the  pulmonary  epithelium 
by  the  simple  process  of  diffusion. 

The  Exchange  of  Carbon  Dioxide. — Analysis  of  venous  blood 
shows  that  100  c.  c.  contains  about  45  to  50  c.  c.  of  carbon  diox- 
ide, and  that  the  gas  exerts  a  pressure  or  tension  of  about  40  mm. 
mercury,  which  is  equal  to  about  five  per  cent  of  an  atmosphere. 
Now  water  will  dissolve  under  these  conditions  about  2i/^  c.  c.  of 
carbon  dioxide  per  100  c.  c.  This  would  leave  the  most  of  the 
carbon  dioxide  of  the  blood  unaccounted  for,  in  case  the  blood 
has  the  same  solvent  power  for  the  gas  that  water  has.  The  rest 
of  the  carbon  dioxide  therefore  must  be  accounted  for  as  being 
in  chemical  combination  with  the  constituents  of  the  plasma  and 
corpuscles.  The  major  part  is  probably  held  in  the  form  of 
sodium  carbonate  and  bicarbonate,  the  remainder  being  combined 
with  the  proteins  of  the  plasma  and  the  red  corpuscles.  The  most 
satisfactory  explanation  of  the  manner  in  which  carbon  dioxide 
is  dissociated  from  the  above  mentioned  compounds  in  the  blood, 
is  that  there  are  substances  in  the  plasma,  such  as  the  blood  pro- 
teins, which  act  as  weak  acids,  and  gradually  drive  off  the  carbon 
dioxide  when,  as  in  the  air  in  the  lungs,  its  escape  is  rendered 
easy  by  a  lowered  carbon  dioxide  pressure  outside  the  plasma. 


CHAPTER  XXII. 

THE  RESPIRATION  (Cont'd). 

The  External  Respiration. 

Anatomical  Considerations. — The  constant  call  of  the  tissues 
for  oxygen  and  the  formation  of  the  waste  gas,  carbon  dioxide, 
demands  a  mechanism  by  which  the  blood  can  continually  renew 
its  supply  of  oxj^gen  and  excrete  its  excess  of  carbon  dioxide. 
This  exchange,  as  we  have  seen,  is  effected  in  the  lungs,  which 
are  built  up  in  the  following  way : . 

The  nasal  and  oral  cavities  lead  to  the  pharynx,  from  which 
open  two  tubes:  one  posterior,  the  oesophagus,  going  to  the  ali- 
mentary tract,  and  the  other,  anterior  the  trachea,  going  to  the 


Fig.    28. — Diagram    of   structure   of    lungs   showing    larynx,    bronchi,    bron- 
chioles and  alveoli. 

lungs  (Fig.  28).  At  the  beginning  of  the  trachea  is  placed  the 
larynx,  or  the  voice  box,  the  opening  of  which  is  guarded  by  a 
flap  of  tissue,  the  epiglottis.  Within  the  larynx  are  the  vocal 
cords.  The  trachea,  or  windpipe,  is  a  relatively  large  tube,  about 
four  and  one-half  inches  long,  which,  after  its  entrance  into  the 
thorax,  divides  into  two  tubes,  the  bronchi,  each  of  which  subdi- 
vides again  and  again,  the  branches  gradually  growing  smaller 
until  they  are  mere  twigs,  and  are  known  as  bronchioles,  or  small 

207 


208  HUMAN  PHYSIOLOGY. 

bronchi.  The  lumen  of  the  trachea  and  bronchi  is  maintained 
patent  by  cartilage  plates,  which  are  imbedded  in  the  walls  of  the 
tubes.  The  bronchioles,  however,  have  no  such  plates,  their  walls 
being  composed  of  fibrous  and  elastic  tissue,  in  which  is  a  layer 
of  smooth  muscle.  The  whole  system  of  tubes  is  lined  with  a 
layer  of  ciliated  epithelium. 

The  bronchioles  terminate  in  wide  air  sacs  or  cavities,  the  in- 
fundibuli,  from  the  walls  of  which  extend  numerous  minute  cavi- 
ties, the  alveoli.  The  walls  of  the  alveoli  are  very  thin  but 
strong,  and  are  composed  of  a  layer  of  elastic  tissue  lined  with  a 
single  layer  of  flattened  epithelium.  It  is  estimated  that  the  epi- 
thelial surfaces  of  the  alveoli,  if  they  were  spread  out  on  a  flat 
surface,  would  cover  about  1,000  square  feet.  Such  a  large  area 
exposed  to  the  air  of  the  lungs  offers  the  best  of  facilities  for  the 
rapid  exchange  of  the  respiratory  gases,  and  in  fact  the  walls  of 
the  alveoli  are  the  true  respiratory  membrane  of  the  lung,  for 
through  them  the  exchange  of  gases  between  the  air  and  the 
blood  takes  place.  Below  the  epithelial  cells  of  the  alveoli  lie  the 
capillaries  of  the  pulmonary  artery  in  a  regular  meshwork;  so 
numerous,  indeed,  are  they  that  each  individual  erythrocyte  is 
able  to  come  in  close  contact  with  the  air  in  the  alveolus,  separ- 
ated only  therefrom  by  the  lining  of  the  alveolus,  the  wall  of  the 
artery,  and  the  plasma  of  the  blood.  This  arrangement  makes 
possible  the  rapid  exchange  of  gases  which  must  take  place  with- 
in the  lungs.' 

The  two  lungs  in  company  with  the  heart  occupy  the  thoracic 
cavity,  which  is  bounded  above  and  on  the  sides  by  the  ribs  and 
their  attached  tissues,  and  below  by  the  diaphragm,  a  muscular 
sheet  of  tissue  which  divides  the  body  cavity  into  a  thoracic  and 
an  abdominal  portion  (Fig.  29).  The  lungs  are  suspended  at 
their  roots,  which  are  composed  of  the  trachea  and  the  pulmonary 
blood  vessels,  and  they  lie  free  in  the  thoracic  cavity  in  close  ap- 
position with  the  walls  of  the  thorax.  Covering  the  outside  of 
the  lungs  and  the  inside  of  the  thoracic  cavity,  which  is  in  con- 
tact with  the  lungs,  is  a  thin  endothelial  membrane  known  as  the 
pleura,  the  surface  of  which  is  kept  moist  by  a  secretion  of 
lymph.     This  smooth  membrane  allows  the  surface  of  the  lungs 


THE  MECHANISM  OF  BREATHING. 


209 


to  move  easily  over  the  inner  surface  of  the  thorax  during  the 
changes  in  the  size  of  the  cavity  which  accompany  the  respiratory 
movements. 

Mechanism  of  Breathing-. — Normal  breathing  has  the  object 
of  bringing  about  a  constant  renewal  of  air  in  the  lungs,  and  it 
is  effected  by  movements  of  the  thorax  and  diaphragm.  When- 
ever the  cavity  of  the  thorax,  is  enlarged,  as  in  the  act  of  inspira- 
tion, the  lungs  must  increase  in  size  to  fill  the  space,  and  air  is 


Fig.  29. — The  position  of  the  lungs  in  the  thorax.      (T.  Wingate  Todd.) 


pushed  into  the  respiratory  tubules  and  the  air  sacs  by  the  pres- 
sure of  the  outside  atmosphere.  At  expiration  the  reverse  takes 
place,  and  the  air  is  expelled.  A  very  good  conception  of  the 
mechanism  by  which  this  is  brought  about  may  be  had  by  refer- 
ence to  Fig.  30.  Any  increase  in  size  of  the  bottle,  as  by  pulling 
down  the  bottom  rubber  membrane,  will  cause  air  to  expand  the 
rubber  sacs  coming  in  by  the  tube  passing  through  the  cork  of  the 
bottle.  When  the  size  of  the  cavity  is  decreased  by  releasing  the 
membrane,  the  reverse  takes  place  and  air  is  expelled  from  the 
rubber  sacs. 

With  every  inspiration  the  thorax  is  increased  in  size  in  all 


210 


HUMAN  PHYSIOLOGY. 


diameters,  from  above  downwards  by  the  contraction  of  the 
diaphragm,  and  in  the  transverse  diameter  by  the  movement  of 
the  ribs. 

The  Part  Played  by  the  Diaphragm. — The  diaphragm  is  a 
circular  sheet  of  muscle  which  divides  the  body  cavity  into  two 
compartments,  the  upper  being  the  thorax,  the  lower  the  abdom- 


Fig.  30. — Hering-'s  apparatus  for  demonstrating  the  action  of  the  respira- 
tory pump.  The  thorax  is  represented  by  a  bottle,  the  diaphragm  by  a  sheet 
of  rubber  forming  its  bottom,  the  trachea  by  a  tube  passing  through  the 
corli,  and  the  lungs  by  two  thin  rubber  bags.  A  thin  piece  of  rubber  tubing 
crosses  the  bottle.  This  represents  the  heart.  The  action  of  the  diaphra.^^m 
pumps  air  in  and  out  of  the  lungs  and  water  through  the  heart.  The  lungs 
and  heart  are  thin  rubber  bags.      (From  Baird  and  Co.'s  catalogue.) 


inal  cavity.  In  the  upper  compartment  are  the  lungs  and  heart 
with  the  accompanying  blood  vessels  and  air  passages.  The  ab- 
dominal cavity  contains  the  digestive  organs  and  glands,  as  the 
liver,  kidneys,  spleen  and  reproductive  organs.  The  peripheral 
edges  of  the  diaphragm  are  attached  to  the  lumbar  vertebrae  at 
the  back,  to  the  lower  border  of  the  ribs  on  the  sides,  and  to  the 
tip  of  the  sternum  in  front.     The  muscular  fibers  radiate  to- 


THE  MECHANISM   OF  BREATHING. 


211 


wards  the  center  and  end  in  a  tendinous  sheet  of  tissue  called  the 
central  tendon  of  the  diaphragm.  When  these  fibers  are  relaxed, 
the  diaphragm  is  pushed  up  into  the  thoracic  cavity,  forming  a 
dome-shaped  arcli.  This  is  caused  by  the  pressure  of  the  abdomi- 
nal organs,  supported  by  the  muscular  walls  of  the  abdomen,  on 
its  lower  surface,  a  suction  pressure  on  the  upper  surface  of  the 
diaphragm  being  maintained  by  the  natural  tendency  of  the 
lungs  to  contract.  The  central  tendon  is  pulled  downwards  and 
the  arched  dome  is  flattened  on  contraction  of  the  diaphragm, 


Fig.    31. — Diagram    to   show    movement   of   diaphragm    during    respiration : 
I,   expiration  ;   II,   normal   inspiration ;   III,   forced   inspiration. 


thus  increasing  the  size  of  the  thoracic  cavity  (Fig.  31).  An- 
other result  of  the  lowering  of  the  diaphragm  is  the  slight  pro- 
trusion of  the  abdomen  due  to  the  pressure  exerted  on  the  vis- 
cera. This  type  of  breathing  is  therefore  known  as  abdominal  or 
diaphragmatic  breathing. 

The  Part  Played  by  the  Thorax. — The  action  of  certain 
muscles  attached  to  the  ribs  also  produces  an  enlargement  of  the 
thoracic  cavity.    Each  pair  of  corresponding  ribs,  which  are  ar- 


212  HUMAN   PHYSIOLOGY. 

ticiilated  posteriorly  with  the  vertebral  column  and  anteriorly 
with-  the  sternum,  forms  a  ring  directed  obliquely  from  behind 
forwards  and  downwards.  Any  muscles  whose  action  would 
bring  about  a  raising  of  the  anterior  ends  of  the  ribs,  would 
therefore  lessen  the  oblique  position  and  increase  the  distance  be- 
tween each  pair  of  ribs,  and  also  add  to  the  antero-posterior 
diameter  of  the  thorax.  Each  rib  increases  in  length  from 
above  downwards,  and  as  the  ribs  are  raised,  the  lower  longer 
rib  occupies  the  place  previously  held  by  its  shorter  neighbor. 
This  movement  therefore  causes  the  dome  or  apex  of  the  thorax 
to  become  more  flat  and  broad.  And  also  the  lower  ribs  are 
so  articulated  with  the  spinal  column  that  they  exhibit  an  up- 
ward rotary  movement,  which  resembles  that  made  by  a  bucket 
handle,  and  which  increases  the  lateral  or  transverse  diameter 
of  the  thorax. 

The  muscles  which  are  responsible  for  the  inspiratory  eleva- 
tion of  the  ribs  are  mainly  the  external  intercostals,  aided  by 
other  muscles  of  the  thorax,  some  of  which  are  called  into  use 
only  when  very  powerful  respiratory  movements  are  necessary. 

Normal  expiration  is  almost  entirely  a  passive  act.  The  re- 
coil of  the  stretched  elastic  tissue  of  the  lungs,  after  the  in- 
spiratory muscles  have  ceased  to  act,  returns  the  diaphragm  and 
thoracic  cage  to  the  expiratory  position.  This  is  aided  somewhat 
by  the  actions  of  the  internal  intercostal  muscles  which  lower  the 
ribs.  By  increasing  the  size  of  the  thoracic  cavity,  inspiration 
causes  a  corresponding  increase  in  volume  of  the  thoracic  organs ; 
viz.,  the  lungs  and  the  vascular  structures,  because  the  thorax  is 
a  closed  cavity,  and  whenever  it  expands  it  must  either  produce  a 
vacuum  between  the  organs  which  fill  it  and  its  own  walls,  or  the 
volume  of  the  organs  must  increase.  It  is  the  latter  process 
which  mainly  occurs,  the  result  being  that  air  is  pushed  into  the 
lungs  by  the  atmospheric  pressure  whenever  the  thoracic  cavity 
is  increased  in  size. 

The  Movements  op  the  Lungs. — The  changes  produced  in 
the  size  of  the  thoracic  cavity  and  the  lungs  during  normal  res- 
piration or  in  disease,  are  easily  determined  by  noting  the  sounds 
which  are  produced  by  tapping  or  percussing  with  the  fingers  the 


THE  MECHANISM   OP  BREATHING.  213 

thoracic  walls  during  inspiration  and  expiration.  A  low-pitched 
resonant  sound  is  elicited  over  the  lungs  containing  air,  whereas 
a  high-pitched  non-resonant  or  tympanitic  hollow  sound  is  heard 
over  the  solid  viscera  and  abdominal  organs.  In  disease  when 
changes  take  place  in  the  substance  of  the  lungs,  as  in  tubercu- 
losis, pneumonia,  etc.,  alterations  occur  in  the  tone  elicited  on 
percussion.  These  alterations  are  of  great  diagnostic  import- 
ance. In  pleurisy,  a  condition  in  which  the  pleural  surfaces  are 
roughened,  a  friction  rub  or  vibration,  produced  by  the  rubbing 
of  the  roughened  surfaces  of  the  pleura  of  the  lungs  on  that  of 
the  thorax,  may  be  detected  by  placing  the  ear  over  the  affected 
area.  The  pain  following  a  broken  rib  is  caused  by  the  irritation 
of  the  pleural  membrane  by  the  broken  edge  of  the  rib.  It  is  al- 
leviated by  making  the  ribs  immovable  by  tightly  strapping  the 
thorax  with  adhesive  plaster  over  the  region  of  the  pain. 

Respiratory  Sounds. — Accompanying  inspiration  a  rustling 
sound,  described  as  a  vesiculcir  sound,  may  be  heard  over  most  of 
the  lung  area.  It  is  produced  by  the  dilatation  of  the  alveoli  and 
fine  bronchi.  Over  the  larger  air  passages  a  high,  sharper  tone 
is  heard,  called  bronchial  breathing.  In  diseases  in  which  the 
alveoli  are  destroyed  and  the  lungs  are  filled  with  fluid,  etc., 
the  bronchial  breath  sounds  replace  the  vesicular  sounds. 

Effect  of  Respiration  on  the  Movement  of  the  Blood  and  on 
Blood  Pressure. — Within  the  thorax  the  changes  in  pressure 
accompanying  each  respiration  affect  the  heart  and  so  influence 
somewhat  the  movement  of  the  blood.  In  thin  individuals  it  is 
easy  to  confirm  this  by  observing  the  effect  of  breathing  on  the 
blood  flow  through  the  jugular  vein.  At  each  inspiration  the 
jugular  vein  is  seen  to  empty,  and  during  expiration  to  fill.  If 
simultaneous  records  are  taken  of  the  blood  pressure  and  re- 
spiratory movements  in  ordinary  breathing,  it  will  generally  be 
observed  that  during  inspiration  there  is  a  rise  and  during  ex- 
piration a  fall  of  blood  pressure.  .  This  phenomenon  is  explained 
as  follows :  During  inspiration  the  heart  is  better  supplied  with 
blood  and  can  fill  more  quickly  and  perfectly  than  during  ex- 
piration, because  the  decrease  in  the  pressure  in  the  thorax  at 
this  period  serves  to  accelerate  the  movement  of  venous  blood 


214  HUMAN  PHYSIOLOGY.  ^ 

into  the  thorax  by  expanding  the  larger  veins.  The  expansion 
of  the  lungs  at  inspiration  also  dilates  the  capillaries  and  arteri- 
oles imbedded  in  these  tissues,  hence  a  greater  volume  of  blood 
can  pass  through  them  in  the  same  unit  of  time.  If  the  heart 
beat  remains  constant  in  strength  and  rate,  the  increased  amount 
of  blood  pumped  during  inspiration  will  cause  the  blood  pres- 
sure to  rise. 

It  is  well  to  bear  in  mind  that  under  abnormal  conditions  the 
respiration  may  affect  the  blood  pressure  to  a  dangerous  extent. 
For  instance,  in  the  attempt  to  force  air  from  the  lungs  under 
pressure  into  a  vessel,  as  in  blowing  up  a  football  or  testing  the 
strength  *of  expiration  on  a  machine  made  for  the  purpose,  the 
air  pressure  can  be  increased  within  the  thorax  to  more  than 
equal  the  pressure  in  the  vessels  of  the  lungs,  and  the  circulation 
is  temporarily  stopped  in  the  pulmonary  vessels.  The  blood  be- 
comes dammed  up  in  the  venous  system  and  forced  out  of  the 
lungs  by  the  pressure  of  air.  This  experiment  is  dangerous  in 
one  who  has  not  a  first-class  heart  and  vascular  system.  The  ef- 
fects on  the  lungs  and  blood  pressure  of  sucking,  inspiration  and 
expiration  can  be  conveniently  reproduced  on  an  artificial  schema 
which  represents  the  thoracic  cavity,  lungs,  heart  and  related 
vessels,  as  shown  in  Fig.  31. 

Variations  occur  in  the  respiratory  movements  under  various 
emotional  and  physical  conditions.  Any  foreign  or  irritating 
body  within  the  air  passages  will  cause  a  cough.  This  consists 
in  a  forced  expiration,  during  the  first  portion  of  which  the  glot- 
tis is  closed.  The  irritating  substance  is  likely  to  be  .expelled  by 
the  sudden  opening  of  the  glottis.  The  presence  of  irritating 
substances  in  the  nasal  cavity  gives  rise  to  sneezing,  a  sudden 
and  noisy  expiration  through  the  nasal  passages  preceded  by  a 
rapid  and  deep  inspiration.  In  crying,  inspirations  are  short  and 
spasmodic,  followed  by  prolonged  expirations,  whereas  laughing 
is  quite  the  reverse.  Yawning,  the  expression  of  drowsiness  or 
ennui,  consists  in  long  deep  inspirations  followed  by  a  short  ex- 
piration. Hiccougliing  is  due  to  spasmodic  contractions  of  the 
diaphragm,  the  peculiar  sound  being  due  to  sudden  closure  of 
the  glottis. 


ARTIFICIAL  RESPIRATION. 


215 


Artificial  Respiration. — In  cases  of  suspended  respiration  in 
human  beings  caused  by  drowning,  excess  of  anaesthesia,  or  other 
injury,  artificial  respiration  is  often  necessary  to  restore  normal 
breathing.  The  most  efficient  of  these  methods  is  described  by 
Schafer,  and  is  known  as  Schafer's  method  (Fig.  32).  He  de- 
scribes the  method  as  follows :  It  consists  of  laying  the  subject 
in  the  prone  posture,  preferably  on  the  ground,  with  a  thick 
folded  garment  underneath  the  chest  and  epigastrium.  The 
operator  puts  himself  athwart  or  at  the  side  of  the  subject,  facing 


■ 

f^ 

■ 

■ 

l^^^^^4 

■      K^' 

^^^H 

^^^^^^^H 

W^^' 

HTw.^! 

^^1 

^^^^^H 

L 

/r^iJii 

E 

B  fif^^^^^^^M 

^w 

v-u3 

^^^ 

^^^^1 

Fig.     32. 
(Schafer.) 


-Position     to     be     adopted     for     effecting     artificial     respiration. 


his  head  (Fig.  32)  and  places  his  hands  on  each  side  over  the 
lower  part  of  the  back  (lower  ribs).  He  then  slowly  throws  the 
weight  of  his  body  forward  to  bear  upon  his  own  arms,  and  thus 
presses  upon  the  thorax  of  the  subject  and  forces  the  air  out  of 
the  lungs.  This  being  effected,  he  gradually  relaxes  the  pressure 
by  bringing  his  own  body  up  again  to  a  more  erect  position,  but 
without  moving  his  hands.  These  movements  are  repeated  reg- 
ularly at  a  rate  of  twelve  to  fifteen  per  minute  until  normal  res- 
piration begins. 

Volumes  of  Air  Respired. — At  each  inspiration  the  lungs  take 
in  about  oOO  c.  e.  of  air,  which  is  given  out  again  at  expiration. 
This  is  known  as  the  tidal  air.  After  the  completion  of  the  ordi- 
nary inspiration,  it  is  possible  by  a  forced  inspiration  to  take 


216  HUMAN  PHYSIOLOGY. 

1500  c.  c.  more  air  into  the  lungs.  This  amount  is  known  as  the 
complemental  air.  Likewise  after  a  normal  expiration  about  1500 
c.  c.  more  air  can  be  expelled  from  the  lungs.  This  is  known  as 
the  supplemental  air.  In  spite  of  forced  expiration  there  will 
still  remain  within  the  lung  about  1000  e.  c.  of  air  which  fills  the 
alveoli  and  air  tubes,  known  as  the  residual  air.  This  air  remains 
in  the  air  spaces  after  the  forced  expiration  because  the  lungs 
cannot  relax  to  their  fullest  extent,  being  held  open  by  the  suc- 
tion pressure  of  the  thorax.  In  other  words,  the  thoracic  cavity 
is  larger  in  the  expiratory  position  by  1000  c.  c.  than  the  lungs 
are.  That  this  is  the  case  is  shown  by  the  immediate  contraction 
of  the  lungs  into  a  small  volume  when  the  thorax  is  opened,  for 
then  the  atmospheric  pressure  becomes  equalized  on  the  out- 
side and  inside  of  the  lungs,  and  the  elastic  tissue  contracts  and 
forces  out  the  residual  air.  From  this  it  is  obvious  that  the  elas- 
tic recoil  of  the  stretched  lungs  must  always  tend  to  pull  the 
organ  away  from  the  chest  wall  and  thus  create  a  negative  or  suc- 
tion pressure  within  the  thoracic  cavity.  Anything  which  de- 
stroys this  relation  makes  breathing  impossible,  because  the  lungs 
are  no  longer  held  against  the  chest  walls.  It  is  for  this  reason 
that  wounds  in  the  chest  are  very  dangerous. 

The  trachea,  bronchi,  etc.,  require  quite  a  little  air  to  fill  them, 
so  that  only  a  part  of  the  tidal  air  reaches  the  alveoli.  In  other 
words,  it  is  only  a  portion  of  the  air  we  expire  that  comes  in 
contact  with  the  respiratory  epithelium  and  undergoes  any 
change  in  composition. 

It  is  estimated  that  about  140  c.  c.  represents  the  actual  vol- 
ume of  the  air  tubes.  This  leaves  360  c.  c.  of  air  which  reaches 
the  alveoli.  This  amount  is  used  to  dilute  the  1000  c.  c.  of 
residual  air  and  1500  c.  c.  of  supplemental  air  already  in  the 
alveoli.  In  fact  the  function  of  breathing  may  be  said  to  con- 
sist in  continually  diluting  the  alveolar  air  with  a  quantity  of 
fresh  air  in  order  that  its  composition  may  remain  more  or  less 
constant. 

The  inspired  or  atmospheric  air  is  a  mixture  of  oxygen,  car- 
bon dioxide  and  nitrogen,  and  is  relatively  constant  under  ordi- 
nary conditions.     The  expired  .air  varies  somewhat  according  to 


Oxygen 

CO, 

20.96 

0.04 

16.02 

4.38 

GASEOUS  EXCHANGE  IN  LUNGS.  217 

the  rate  and  depth  of  respiration.    The  following  table  gives  the 
average  percentage  composition  of  inspired  and  expired  air: 

Nitrogen 

Inspired  air 79 

Expired  air  79  + 

The  above  analysis  shows  that  there  is  a  marked  difference  be- 
tween the  inspired  and  the  expired  air.  It  shows  us  further  that 
of  the  oxygen  taken  up  by  the  blood,  only  part  appears  again 
combined  with  carbon  in  the  gas  CO,.  The  retention  of  oxygen 
is  due  to  the  oxidation  of  substances  which  do  not  appear  in  the 
expired  gases.  This  subject  is  fully  discussed  under  the  head  of 
respiratory  quotient  in  the  chapter  on  metabolism  (p.  91). 

These  observations  do  not  enable  us  to  decide  whether  the  laws 
of  diffusion  of  gases  apply  to  the  gaseous  exchange  of  the  lungs. 
To  do  this  we  must  know  the  actual  pressures  of  the  respiratory 
gases  in  the  venous  blood  coming  to  the  lungs  and  in  the  air  of 
the  alveoli.  Many  types  of  experiments  have  been  devised  to  ob- 
tain these  values,  and  although  the  actual  figures  vary  somewhat 
in  the  hands  of  different  investigators,  the  results  as  a  whole  in- 
dicate that  the  gaseous  exchange  of  the  lungs  is  dependent  solely 
on  the  presence  of  a  higher  pres.sure  of  oxygen  and  a  lower  pres- 
sure of  carbon  dioxide  in  the  alveolar  air  than  are  present  in  the 
blood  coming  to  the  lungs.  The  ability  of  haemoglobin  to  take  up 
oxygen  with  great  readiness  at  oxygen  pressures  which  exceed 
50  or  60  mm.  mercury  pressure  indicates  that  the  blood  can  still 
obtain  oxygen  from  air  which  contains  only  one-half  of  the  nor- 
mal pressure  of  oxygen.  In  whatever  way  we  estimate  it,  the 
oxygen  pressure  in  the  alveoli  is  always  greater  than  this. 

We  will  not  go  into  details  regarding  the  methods  which  have 
been  employed  in  solution  of  these  problems ;  suffice  it  to  say  that 
a  very  fair  sample  of  alveolar  air  can  be  secured  by  collecting  a 
sample  of  air  from  a  tube  through  which  a  forced  expiration  has 
been  made.  The  last  portions  of  such  expired  air  must  obviously 
be  alveolar  air. 

Mechanism  of  Gaseous  Exchange  in  Lungs. — We  have  seen 
that  in  the  blood  the  pressure  or  tension  of  the  oxygen  is  greater, 


218 


HUMAN   PHYSIOLOGY. 


whereas  that  of  the  COo  is  less  than  in  the  tissues.  These  rela- 
tions will  account  for  the  gas  exchange  which  occurs  between  the 
blood  and  tissues  if  we  apply  the  physical  law  of  the  diffusion  of 
gases,  which  states  that  two  gases  under  different  pressures  and 
separated  by  a  membrane  through  which  they  may  pass  free- 
ly, will  mix  with  each  other  until  the  tensions  on  both  sides 
of  the  membrane  are  equal.  Before  this  law  can  be  applied  to 
explain  the  exchange  of  gases  between  the  blood  and  air  within 
the  lungs^  we  must  prove  that  the  tension  of  the  oxygen  is  less, 
and  of  the  CO2  greater  in  the  venous  blood  than  in  the  alveolar 
air.  A  consideration  of  these  problems  is  included  under  the 
subject  of  external  respiration. 


CHAPTER  XXIII. 
THE  RESPIRATION  (Cont'd). 

Nervous  Control  of  Respiration. — Under  normal  conditions 
we  breathe  from  14  to  18  times  a  minute.  According  to  the  de- 
mand of  the  tissues  for  oxygen,  we  breathe  fast  or  slow,  but  the 
respirations  are  rhythmic  in  time  and  under  like  conditions  are 
equal  in  volume.  The  respiratory  movements,  unlike  those  of 
the  heart  are  initiated  by  impulses  transmitted  to  the  respira- 
tory muscles  from  the  central  nervous  system.  These  arise  from 
the  so-called  respiratory  centers  in  the  medulla  oblongata  (p. 
256).  Anatomically  these  centers  cannot  be  sharply  localized, 
but  destruction  of  the  portion  of  the  medulla  in  which  they  exist 
causes  an  immediate  cessation  of  respiratory  movements.  The 
centers  are  connected  with  the  diaphragm  by  the  phrenic  nerves, 
and  to  the  muscles  of  the  ribs,  larynx  and  nares  by  spinal  or 
cranial  nerves.  Like  all  other  nerve  centers,  the  respiratory 
center  is  influenced  by  afferent  impulses,  the  chief  ones  of 
which  come  from  the  lungs  by  way  of  the  vagus,  but  there 
are  many  others.  In  fact  all  the  sensory  nerves  of  the  body, 
as  well  as  the  higher  centers  of  the  brain,  are  able  to  influ- 
ence the  respiratory  center.  Disease  of  the  phrenic  nerves 
causes  paralysis  of  the  diaphragm,  and  impairs  the  ventilation 
of  the  lungs.  Likewise  paralysis  involving  the  spinal  cord  be- 
low the  exit  of  the  phrenic  nerves  may  paralyze  the  nerves 
of  the  thoracic  muscles,  and  throw  the  whole  work  of  respiration 
on  the  diaphragm. 

If  the  vagus  nerves  of  a  dog  or  cat  are  cut  in  the  neck,  the 
respiration  becomes  deeper  and  slower,  yet  the  volume  of  air  re- 
spired per  minute  is  not  greatly  altered.  This  change  is  due  to 
the  elimination  of  stimuli  normally  coming  from  the  lungs  by 
way  of  the  vagi  to  the  respiratory  center,  which  serve  to  control 
the  depth  of  respiration.  It  can  be  experimentally  demonstrated 
that  the  collapse  of  the  alveoli  of  the  lungs  which  occurs  at  the 

219 


220  /  HUMAN  PHYSIOLOGY. 

end  of  normal  expiration,  and  the  stretching  of  the  alveolar  walls 
which  occurs  at  the  end  of  normal  inspiration,  cause  stimuli  to  be 
passed  along  the  vagi  to  the  center,  and  that  these  stimuli  bring 
on  the  next  phase  of  respiration.  The  breaking  of  the  connection 
between  the  lungs  and  the  alveoli  destroys  this  influence  and  the 
respirations  become  deep  and  slow. 

In  the  absence  of  the  vagi,  the  higher  centers  assume  partial 
control  of  the  regulation  of  the  respiratory  movements.  If  they 
also  are  destroyed,  however,  breathing  becomes  inadequate  to 
maintain  life,  although  the  center  itself  is  still  able  to  keep  up  a 
modified,  rhythmic  respiration. 

Reflex  Respiratory  Movements. — The  cutaneous  nerves,  es- 
pecially those  of  the  face  and  abdomen,  have  a  marked  influence 
on  respiration.  These  can  be  excited  by  heat  or  cold  or  pain; 
for  instance,  a  cold  bath  will  cause  a  deepening  or  quickening  of 
the  respiration.  Another  example  is  found  in  the  forced  ex- 
piratory effort  made  on  inhalation  of  acid  or  sharp  smelling  sub- 
stances, which  not  only  affect  the  olfactory  nerves,  but  also  the 
sensitive  endings  of  the  fifth  nerve  in  the  nasal  mucous  mem- 
brane. 

Chemical  Control  of  Eespiration. — ^In  spite  of  this  very  effec- 
tive method  of  nervous  control  of  the  respiration,  there  is  an- 
other no  less  important  means  of  respiratory  control,  which  de- 
pends on  the  ability  of  chemical  substances  in  the  blood  to  stim- 
ulate the  respiratory  center.  The  substances  which  most  readily 
affect  the  center  are  acids,  such  as  carbon  dioxide  (which  in  solu- 
tion forms  a  weak  acid,)  and  lactic  acid,  which  is  formed  under 
certain  conditions  in  the  body.  Lack  of  oxygen,  if  it  be  consid- 
erable, also  causes  the  center  to  show  marked  signs  of  activity. 
In  the  introductory  chapter  the  physico-chemical  properties  of 
the  blood  and  tissue  fluids  were  discussed.  It  will  be  recalled 
that  these  are  practically  neutral  fluids,  that  is,  they  show  an  al- 
most exact  balance  in  the  number  of  hydrogen  and  hydroxyl 
ions,  a  condition  which  determines  the  neutrality  of  a  fluid.  Any 
increase  in  the  amount  of  carbon  dioxide  in  the  blcfod  would  form 
proportionately  more  carbonic  acid,  which  yields  hydrogen  ions, 
and  thus  tend  to  destroy  the  neutral  balance  of  the  blood.    This 


CHEMICAL   CONTROL   OF   RESPIRATION.  221 

increase  in  the  hydrogen  ion  concentration  in  the  blood  is  suffi- 
cient to  stimulate  the  respiratory  center  and  augment  the  rate 
and  depth  of  respiration  in  order  to  expel  the  carbon  dioxide 
and  thus  reduce  the  acidity  of  the  blood.  All  acids  which  yield 
hydrogen  ions  in  solution  have  this  effect  on  respiration  when 
they  are  injected  into  the  blood.  Lactic  acid,  which  is  formed 
when  the  oxygen  supply  to  the  tissues  is  diminished  or  inade- 
quate, is  perhaps  the  most  important  factor  coming  into  play  in 
the  stimulation  of  the  respiratory  center  which  occurs  during 
exercise.  The  carbon  dioxide  tension  of  the  blood  during  exer- 
cise may  be  actually  decreased  owing  to  the  increased  ventilation 
of  the  lungs  as  a  result  of  the  presence  of  lactic  acid  in  the  blood. 

The  increase  in  breathing  due  to  lack  of  oxygen  is  not  nearly 
so  easily  elicited  as  that  caused  by  excess  of  acids.  In  fact,  the 
percentage  of  oxygen  may  be  diminished  to  about  one-half  of 
that  found  in  the  atmosphere  before  breathing  is  markedly  af- 
fected. 

In  disturbances  of  the  gaseous  exchange  of  the  lungs,  the  re- 
spiratory center  attempts  to  compensate  for  the  change  by  in- 
creasing the  number  and  the  depth  of  the  respirations.  If  the 
gas  exchange  be  markedly  insufficient,  the  breathing  becomes 
very  much  exaggerated,  and  practically  all  possible  respiratory 
muscles  are  called  into  play.  This  is  the  case  during  an  attack 
of  asthma,  in  which  the  muscles  of  the  arms  and  abdomen  are 
used  by  the  patient  in  his  efforts  to  obtain  enough  air.  Difficult 
breathing  of  this  kind  is  known  as  dyspnoea.  If  the  gas  exchange 
is  very  insufficient,  the  phenomenon  of  asphyxia  sets  in. 

The  control  of  the  respiration,  therefore,  may  be  said  to  be 
two-fold,  dependent  not  only  on  the  nerve  supply  of  the  respira- 
tory center  from  the  afferent  sensory  and  cerebral  nerves,  but 
also  on  the  chemical  constitution  of  the  blood,  which  stimulates 
the  center  directly.  Each  factor  plays  an  important  part  in  the 
control  of  the  respiratory  movements. 

The  hroncJiial  muscles  are  supplied  through  the  vagi  with 
nerve  fibers  which  produce  dilatation  and  constriction  of  the 
bronchi.  Just  what  the  normal  conditions  are  which  call  for  the 
action  of  these  nerves  is  not  known.    It  is  generally  thought  that 


222  HUMAN   PHYSIOLOGY. 

asthma  is  caused  by  the  constriction  of  the  bronchioles  by  spasm 
of  the  bronchial  muscles.  Atropin,  a  drug  which  paralyzes  cer- 
tain nerves,  is  of  therapeutic  use  in  this  disease,  since  it  paralyzes 
the  nerve  endings  in  the  bronchial  muscles.  Adrenalin  is  also 
sometimes  of  use. 

The  Eflfeot  of  Changes  in  the  Respired  Air  on  the  Respiration. 
A  very  slight  increase  in  the  percentage  of  carbon  dioxide  in  the 
alveolar  air  is  accompanied  by  a  very  marked  quickening  of  re- 
spiration. On  the  other  hand,  the  carbon  dioxide  content  of  the 
atmosphere  may  be  increased  to  about  one  per  cent  without  em- 
barrassing the  respiratory  function,  except  during  muscular 
work,  and  it  is  only  at  concentrations  of  carbon  dioxide  of  three 
or  four  per  cent  of  an  atmosphere  that  the  respiratory  function 
is  seriously  impaired.  The  reason  for  this  is  that  the  inspired 
air  becomes  greatly  diluted  before  it  reaches  the  alveoli,  so  that 
a  slight  increase — up  to  one  per  cent  of  carbon  dioxide — in  the 
atmosphere  only  quickens  and  deepens  the  respiration  sufficient- 
ly to  maintain  the  pressure  of  carbon  dioxide  at  its  normal  level 
in  the  alveoli. 

An  increase  in  the  oxygen  pressure  has  no  such  effect.  In  fact 
pure  oxygen  has  scarcely  any  influence  on  the  rate  of  breathing 
in  the  normal  man.  In  persons  suffering  with  heart  failure  or 
diseases  in  which  the  respiratory  function  of  the  lungs  is  im- 
paired, however,  the  presence  of  a  high  concentration  of  oxygen 
in  the  alveoli  may  make  it  possible  for  the  oxygen-starved  blood 
to  obtain  enough  of  this  gas  to  saturate  it  q,nd  thus  improve  the 
general  condition.  The  reason  for  these  effects  of  oxygen  is  that 
under  normal  conditions  the  pressure  of  oxygen  in  the  atmos- 
phere is  more  than  sufficient  to  saturate  the  hgemoglobin  of  the 
blood,  so  that  an  increase  in  the  oxygen  pressure  will  add  only  a 
small  amount  more  of  oxygen  to  that  dissolved  in  the  plasma  al- 
ready. On  the  other  hand,  the  oxygen  pressure  in  the  atmos- 
phere may  be  reduced  to  less  than  half  that  found  at  sea  level 
without  destroying  life.  This  brings  up  the  interesting  question 
of  mountain  sickness. 

Mountain  Sickness. — At  an  altitude  of  5,000  meters  (about 
16,000  feet)  the  air  is  reduced  to  a  little  over  half  an  atmos- 


VENTILATION.  223 

phere,  and  the  oxygen  tension  is  therefore  only  about  eleven  per 
cent  of  an  atmosphere  in  place  of  twenty  per  cent.  Therefore, 
in  order  to  supply  the  needed  oxygen,  respiration  must  become 
more  rapid.  This,  however,  by  washing  out  the  carbon  dioxide, 
serves  to  reduce  the  tension  of  carbon  dioxide  in  the  alveoli  and 
blood  to  such  an  extent  that  the  action  of  this  gas  on  the  re- 
spiratory center  is  weakened,  and  breathing  may  be  very  slow 
or  cease  for  a  time,  producing  a  condition  known  as  apnea.  The 
lack  of  oxygen  weakens  the  heart,  the  slightest  muscular  move- 
ments are  accomplished  with  difficulty,  and  the  individual  suf- 
fers from  nausea,  vertigo,  headache  and  general  weakness.  After 
living  for  some  time  at  such  altitudes  a  person  becomes  accus- 
tomed to  the  rarity  of  the  atmosphere-  and  in  some  manner  is 
able  to  compensate  for  the  lessened  oxygen  in  the  air. 

Ventilation. — The  disagreeable  odor  of  a  crowded  room  and 
the  symptoms  which  accompany  it  are  well  known  and  are  usual- 
ly attributed  to  the  rebreathing  of  air.  In  support  of  this  the 
historical  incident  of  the  Black  Hole  of  Calcutta,  in  which  many 
people  perished  from  lack  of  air,  is  often  cited.  We  have  already 
seen  that  atmospheres  up  to  one  per  cent  of  carbon  dioxide,  or 
containing  less  than  half  of  the  normal  percentage  of  oxygen, 
can  be  respired  with  no  ill  effects.  But  the  percentage  of  carbon 
dioxide  in  the  worst  ventilated  room  does  not,  as  a  rule,  rise  above 
five-tenths  per  cent,  or  at  most  over  one  per  cent,  of  an  atmos- 
phere. That  this  amount  affects  our  body  metabolism  is  impos- 
sible, since  the  carbon  dioxide  in  the  alveolar  air  is  kept  at  a 
constant  level  of  from  five  to  six  per  cent  by  the  control  which  the 
respiratory  center  exercises  on  the  respiratory  movements. 
Moreover  perfectly  normal  respiration  can  take  place  in  d  room 
where  the  oxygen  content  is  so  low  that  a  match  will  not  burn. 

Because  of  these  facts  it  was  suggested  at  one  time  that  a  toxic 
substance  might  be  present  in  the  expired  air,  but  this  has  not 
been  confirmed  by  subsequent  investigators.  In  spite  of  the  fact 
that  there  is  a  normal  percentage  of  oxygen  and  carbon  dioxide,  a 
room  may  be  unbearably  close  if  it  is  too  warm  and  the  air  is 
saturated  with  moisture.  So  long  as  the  body  can  radiate  its  heat 
quickly  into  the  atmosphere,  the  room  does  not  feel  stuffy,  but 


224  HUMAN  PHYSIOLOGY. 

when  evaporation  is  slow,  because  of  saturation  of  the  air,  and 
heat  is  no  ionger  given  off  quickly  by  the  body,  the  individuals 
in  the  room  become  very  uncomfortable.  An  electric  fan,  which 
distributes  the  air  evenly  over  the  room  and  thus  quickens  the 
removal  of  the  warm  moist  air  immediately  surrounding  the 
body,  adds  much  to  the  comfort  of  the  person.  In  addition  to 
insisting  upon  fresh  air  in  public  offices  and  private  houses, 
it  is  necessary  that  the  ventilating  engineer  should  pay  heed 
to  something  besides  the  percemtage  of  oxygen  and  tearbon 
dioxide  in  the  room.  He  should  also  direct  his  efforts  towards 
cooling  and  increasing  the  circulation  of  the  air  that  surrounds 
the  bodies  of  the  individuals,  by  setting  the  air  in  motion  by 
means  of  fans.  , 

The  conditions  of  temperature,  the  moisture,  and  the  windless 
atmosphere  found  in  public  rooms  and  homes  diminish  the  heat 
loss  of  the  body  and  thus  the  heat  production,  which  means  that 
the  activity  of  the  occupants  must  be  less.  A  reasonable  tem- 
perature with  a  relatively  low  percentage  of  moisture,  and  ordi- 
nary care  in  providing  fresh  air,  will  maintain  the  proper  hy- 
gienic conditions  of  a  room. 

The  Voice. 

The  voice-producing  mechanism  in  man  consists  of  the  trachea, 
through  which  the  air  is  blown  from  the  lungs ;  the  larynx,  the 
modified  upper  portion  of  the  trachea,  which  contains  the  vocal 
cords;  and  the  pharynx,  and  upper  air  passages.  The  larynx 
forms  the  entrance  into  the  trachea.  It  is  composed  of  a  number 
of  cartilaginous  plates  which  are  united  in  a  manner  to  form  a 
box.  Stretched  from  front  to  back  on  each  side  across  the  upper 
portion  of  the  larynx  are  thin  sharp-edged  membranes,  the  vocal 
cords.  The  attachments  of  the  muscles  to  the  cartilages  and  the 
articulations  of  the  several  cartilages  with  each  other,  are  so  ar- 
ranged as  either  to  tighten  or  loosen  the  tension,  or  increase  or 
decrease  the  opening  between  the  edges  of  the  cords.  The  cleft 
between  the  cords  is  called  the  glottis.  The  length  of  the  vocal 
cords  varies  from  11  to  15  mm.,  being  longer  in  men  than  in 
women  and  children.    Branches  of  the  vagus  and  the  spinal  ac- 


THE  VOICE. 


225 


cessory  nerves  supply  the  muscles  of  the  larynx  with  motor 
nerves.  The  sensory  nerves,  arising  in  the  epithelium  of  the 
larynx,  are  also  branches  of  the  vagus.  Mechanical  stimulation 
of  the  mucous  membrane  of  the  larynx  or  electrical  stimulation 
of  the  superior  laryngeal  nerve  will  cause  a  cough  or  a  forced 
expiratory  movement. 

The  Changes  Which  Occur  in  the  Position  of  the  Vocal  Cords 
during  the  production  of  certain  sounds  may  be  studied  by  the 
use  of  the  laryngoscope,  the  principle  of  which  is  shown  in  Fig. 
33.  The  view  obtained  from  such  an  instrument  is  shown  in 
Figs.  34  and  35.    The  base  of  the  tongue  appears  at  the  top ;  be- 


Lfixryn 


Fig.    33. — Diaf^ram   of    laryngoscope. 


low  this  is  the  edge  of  the  epiglottis,  the  flap  of  tissue  guarding 
the  entrance  to  the  larynx ;  and  below  in  the  middle  line  are  seen 
the  true  vocal  cords  as  white  shining  membranes.  Just  above 
these,  on  either  side,  are  two  pink  flaps  of  tissue,  the  false  vocal 
cords.    These  secrete  a  fluid  which  moistens  the  true  cords. 

The  Production  of  the  Voice. —  If  the  vocal  cords  be  put  in 
a  state  of  tension  and  the  aperture  between  them  be  narrowed, 
causing  them  to  offer  a  resistance  to  the  passage  of  air  issuing 
from  the  lungs,  they  may  be  made  to  vibrate  and  to  produce 
sounds.  It  has  been  experimentally  determined  that  a  pressure 
of  expired  air  of  from  140  to  240  nun.  of  water  is  required  to 


226 


HUMAN  PHYSIOLOGY. 


produce  a  sound  of  the  ordinary  pitch  and  loudness,  while  in 
loud  shouting  much  greater  pressures  are  necessary. 

The  sound  of  the  voice,  like  any  other  sound,  may  vary  in 
pitch,  loudness  and  quality.  The  range  of  pitch  of  the  voice  is 
generally  about  two  octaves,  the  pitch  itself  being  determined 
primarily  by  the  lengths  of  the  cords.  This  accounts  for  the 
high-pitched  voice  of  children,  in  whom  the  cords  are  short,  and 
the  low  pitch  of  the  voice  in  men,  in  whom  they  are  long.     In 


Fig.  34. — Position  of  tile  glottis 
preliminary  to  the  utterance  of 
sound,  rs,  true  vocal  cord ;  ar,  ary- 
tenoid cartilage ;  b,  pad  of  the  epi- 
glottis. (From  Stewart's  Physi- 
ology. ) 


Fig.  35. — Position  of  open  glottis. 
1,  tongue ;  e^  epiglottis ;  ae,  ary-epi- 
glottidean  fold  ;  c^  cartilage  of  Wris- 
berg ;  ar^  arytenoid  cartilage ;  o, 
glottis ;  Vj  ventricle  of  Morgagni ; 
ti,  true  vocal  cord ;  ts,  false  vocal 
cord.      (From  Stewart's  Physiology.) 


singing,  three  registers  can  be  distinguished,  the  head,  middle 
and  chest  registers.  The  deeper  notes  of  the  singer  come  from 
the  chest  register,  and  are  produced  by  the  vibrations  of  the  en- 
tire cords,  whereas  in  the  upper  registers  only  the  inner  edge  of 
the  cords  vibrate. 

The  intensity  or  loudness  of  a  vocal  sound  depends  upon  the 
amplitude  of  the  vibrations  of  the  vocal  cords,  and  this  is  pro- 
portional to  the  strength  of  the  expiratory  blast.  The  pitch  of 
a  note  rises  and  falls  somewhat  with  the  intensity  of  the  pres- 
sure of  the  air,  and  for  this  reason  high  notes  are  usually  loud 
notes.  The  quality  of  the  voice,  like  that  of  a  musical  instru- 
ment, depends  on  the  overtones,  or  harmonics,  that  it  produces. 
For  example,  when  a  stretched  string  is  made  to  vibrate,  it  not 
only  vibrates  as  a  whole,  but  portions  of  it  vibrate  independent- 


SPEECH. 


227 


ly  and  give  off  separate  tones  which  are  known  as  overtones. 
Since  the  tone  which  the  string  produces  by  the  vibration  of  its 
entire  length  is  the  loudest  and  lowest  in  pitch,  it  is  picked  out 
as  the  fundamental  tone.  The  fundamental  tones  of  instru- 
ments may  be  exactly  the  same,  but  the  tones  yet  differ  from  one 
another  because  of  the  number  and  the  intensity  of  the  over- 
tones. 

Speech. 

The  pure,  musical  tones  produced  by  the  vocal  cords  are  modi- 
fied by  changes  in  the  character  of  the  air  passages  above  them. 
The  various  combinations  which  are  produced  give  rise  to  sounds 
which  make  up  speech.  Many  of  the  simple  combinations  are 
found  in  all  languages,  but  every  language  is  characterized  by 
certain  sounds  which  are  peculiar  to  it. 

The  sounds  produced  in  speech  may  be  divided  into  two 
groups,  the  vowels  and  the  consonants.  The  vowel  sounds  are 
continuous  and  are  formed  in  the  lower  air  passages  with  the 
help  of  the  glottis.  The  consonants  are  produced  by  more  or 
less  complete  interruptions  of  the  outflowing  air  in  different 
portions  of  the  vocal  tract. 

All  the  vowels  can  be  produced  in  the  whispered  voice,  that  is, 
they  can  be  produced  without  the  actual  vibration  of  the  vocal 
cords.  The  mouth  cavity,  however,  assumes  the  same  position  in 
the  case  of  the  whispered  vowel  as  it  does  for  the  spoken  vowel. 
By  changing  the  shape  of  the  air  passages,  the  various  vowel 
tones  are  produced.  In  Fig.  36  are  seen  the  various  positions  of 
the  tongue  and  palate  for  the  production  of  the  different  vowels. 
When  vowels  are  being  uttered,  the  soft  palate  closes  the  en- 
trance to  the  nasal  cavity. 

The  consonants  are  named  according  to  the  position  at  which 
the  interruption  of  the  air  current  takes  place.  The  labials  are 
formed  at  the  lips :  p,  b ;  the  dentals,  between  the  tongue  and  the 
teeth:  t,  d.  The  gutterals,  k,  g,  ch,  arise  between  the  posterior 
portion  of  the  arched  tongue  and  the  soft  palate;  and  the  Ger- 
man r  is  produced  with  the  help  of  vibrations  of  the  uvula. 


228 


HUMAN   PHYSIOLOGY. 


Sounds  like  m,  n,  ng,  are  termed  nasal  consonants,  since  they 
are  sounded  through  the  nasal  cavity  (see  Fig.  36). 


iO 


u 


K  -  G  FY  Th 


-   C 


5  - 


L 


W 


Fig.   36. — The  position  of  the  tongue  and  lips  during  the  utterance  of   the 
letters  indicated. 


CHAPTER  XXIV. 

THE  FLUID  EXCRETIONS. 

The  Excretion  of  Urine. 

The  Composition  of  the  Urine. — The  waste  substance  result- 
ing from  the  processes  of  metabolism  in  the  tissues  are  eliminated 
from  the  body  in  a  gaseous,  fluid,  or  solid  state.  With  the  excep- 
tion of  the  carbon  dioxide  and  water  of  the  expired  air,  and  cer- 
tain substances  which  glre  excreted  into  the  intestines  or  appear 
in  the  secretions  of  the  skin  glands,  the  metabolic  products  are 
eliminated  in  the  urine. 

The  composition  of  the  urine  is  therefore  rather  complex  and 
varies  greatly  with  the  nature  of  the  food  and  the  amount  of 
water  taken.  By  careful  analysis  of  the  urine  from  a  number  of 
individuals  on  ordinary  diet,  the  average  amount  of  the  various 
constituents  in  what  may  be  considered  a  normal  urine  can  be 
estimated.  Normal  human  urine  is  a  clear  yellow  fluid,  a  little 
heavier  than  water,  having  a  specific  gravity  of  1.016  to  1.02.  If 
tested  with  litmus  paper  it  usually  shows  an  acid  reaction, 
mainly  due  to  the  presence  of  acid  salts,  such  as  sodium  dihy- 
drogen  phosphates,  but  partly  also  to  acid  substances  derived 
from  proteins.  Herbivorous  animals  secrete  an  alkaline  urine, 
which  is  no  doubt  caused  by  the  presence  of  the  large  amount  of 
alkaline  earths  and  the  relatively  small  amount  of  protein  mat- 
ter in  their  diet.  Human  urine  becomes  alkaline  in  reaction 
when  vegetables  are  the  main  ingredients  of  the  diet. 

The  character  of  most  of  the  urinary  constituents  and  the  man- 
ner by  which  they  are  derived  from  the  foodstuffs  have  been  de- 
scribed in  the  cliajjter  on  inetal)olisni,  and  in  the  following  ac- 
count only  a  brief  review  of  their  physical  and  chemical  nature 
is  necessary. 

The  Organic  Substances  of  the  Ukine. — These  compri.se  a 
number  of  nitrogenous  compounds.     The  following  figures,  ob- 

229 


230  HUMAN  PHYSIOLOGY. 

tained  from  tile  results  of  the  analysis  of  a  number  of  normal 
average  urines,  show  how  the  nitrogen  is  distributed  among  these 
compounds. 

Urea    85  to  90% 

Ammonia    2  to    4% 

Creatinin    3% 

Uric  acid  1  to     2% 

Unclassified  nitrogen  5  to     6% 

TJrea. — From  the  above  figures  it  is  seen  that  the  greater  part 
of  the  nitrogen  eliminated  by  man  appears  as  urea.  The  relative 
amount  of  urea  eliminated  depends  very  largely  on  the  diet,  be- 
ing 90  per  cent  or  more  of  the  total  nitrogen  excretion  on  a  full 
protein  diet,  and  60  per  cent  or  less  during  starvation.  The  total 
amount  excreted  is  about  30  grams  per  100  grams  of  protein  in 
the  diet. 

Chemically  urea  has  the  following  formula: 

NH, 

OC 
\ 

If  prepared  pure  it  forms  long  colorless  needles  or  four-sided 
prisms.  It  is  very  soluble  in  water.  Hot  alkalies,  such  as  sodium 
hydroxide,  decompose  it  into  ammonia  and  carbon  dioxide.  The 
same  reaction  occurs  in  case  of  bacterial  decomposition  by  the 
micrococcus  urea,  and  accounts  for  the  ammoniacal  odor  of  urine 
after  standing  in  the  air.  The  significance  of  urea  in  regard  to 
protein  metabolism  and  the  method  of  its  formation  are  dis- 
cussed on  page  108. 

Ammonia. — This,  combined  with  chlorine  or  other  acid  radi- 
cles, is  normally  found  in  small  amounts  in  the  urine.  It  is  one 
of  the  important  agencies  in  maintaining  the  neutrality  of  the 
tissues,  since  with  acids  it  forms  ammonia  salts,  which  are  neu- 
tral in  reaction  and  which  are  eliminated  in  the  urine. 

Creatinin. — The  amount  of  this  substance  found  in  the  urine 


THE  CHEMISTRY  OP  URINE.  231 

is  very  constant  from  day  to  day,  and  is  independent  of  the  diet. 
It  is  largely  a  product  of  the  metabolism  of  the  body  tissues. 

Uric  Acid. — Uric  acid  is  a  purine  body  and  its  relationship 
to  the  other  purines,  and  its  mode  of  formation  and  significance 
are  fully  discussed  in  the  chapter  on  metabolism  (p.  110).  It  is 
relatively  insoluble  in  water,  and  when  allowed  to  crystalize  it 
forms  small  rhombic  crystals.  It  can  unite  with  an  alkali,  such 
as  sodium  hydroxide,  to  form  two  salts :  a  neutral  or  diurate  of 
sodium  (CsHoN^OgNao)  and  the  biurate  or  acid  urate  of  sodium 
(CsHaN^OoHNa).  The  hiurates  are  neutral  in  reaction  and  con- 
stitute the  urates  normally  found  in  the  blood  and  urine.  They 
exist  in  two  isomeric  forms  (a  and  the  h).  The  b  is  more  solu- 
ble than  the  a  form.  It  may  be  that  the  deposition  of  urate  tar- 
tar on  the  teeth,  and  the  deposits  of  urates  in  the  joints  of  a  pa- 
tient suffering  with  gout,  are  due  to  the  change  of  the  h  form 
into  the  less  soluble  a  type. 

There  are  a  number  of  other  nitrogenous  bodies  in  the  urine 
which  are  included  in  the  item  of  unclassified  nitrogen  in  the 
above  analysis.  The  most  important  of  these  is  urinary  indican, 
which  is  derived  from  the  indol  produced  in  the  intestines  by  the 
action  of  bacteria  on  the  amino  acid  tryptophane.  The  yellow 
color  of  the  urine  is  produced  by  a  pigment  called  urochrome, 
which  is  believed  to  be  derived  from  the  pigments  in  the  blood. 

The  Inorganic  Constituents  op  the  Urine. — The  urinary 
salts  are  chiefly  the  chlorides,  sulphates  and  phosphates  of  so- 
dium, potassium,  calcium  and  magnesium.  The  potassium  and 
sodium  salts  are  found  in  greatest  abundance,  since  they  form 
the  main  inorganic  constituent  of  the  food,  and  moreover  the 
greater  portion  of  the  salts  of  the  heavier  metals,  as  calcium,  iron, 
bismuth,  mercury,  etc.,  is  excreted  by  the  intestines.  There  is 
very  little  retention  of  salts  by  the  body  except  during  the  for- 
mation of  bone,  so  that  the  amount  of  the  inorganic  constituents 
of  urine  varies  from  day  to  day  with  the  diet.  The  chlorides  are 
formed  for  the  most  i)art  from  the  inorganic  clilorides  of  the 
food;  the  phosphates  and  the  sulphates  are  derived  from  the  sul- 
phur and  phosphorus  of  the  nucleo-protein  molecules.  If  the 
urine  is  neutral  or  alkaline  in  reaction,  there  is  apt  to  be  a  de- 


232  HUMAN   PHYSIOLOGY. 

posit  of  calcium  or  magnesium  phosphate.     This  will  dissolve 
when  the  urine  is  rendered  faintly  acid. 

Abnormal  Constituents  of  the  Urine. — Many  of  the  sub- 
stances found  in  the  blood  occur  in  minute  traces  in  the  urine. 
When  any  of  these  bodies  are  increased  to  an  unusual  amount 
in  the  urine,  they  become  what  we  may  term  pathological  con- 
stituents. The  bodies  most  commonly  affected  are  the  proteins 
and  sugars.  The  finding  of  a  protein,  such  as  albumin,  in  more 
than  the  faintest  trace,  is  an  indication  of  nephritis  or  Briglit's 
disease.  The  presence  of  albumin  may  be  detected  by  heating  in 
a  test  tube  a  slightly  acidulated  sample  of  urine. 

Normal  urine  contains  the  faintest  trace  of  the  blood  sugar 
dextrose,  but  in  abnormal  conditions,  as  in  the  disease  diabetes 
or  after  a  meal  rich  in  sugars,  a  large  amount  of  dextrose  ap- 
pears in  the  urine  as  a  result  of  an  increase  in  the  sugar  of  the 
blood.  The  condition  probably  represents  the  inability  of  the 
tissues  to  make  use  of  their  carbohydrate  food  in  the  proper  man- 
ner, and  the  kidney  therefore  excretes  the  sugar  as  if  it  were  a 
waste  material. 

The  Organs  of  Excretion,  The  Kidneys. 

Lying  upon  the  posterior  wall  of  the  abdominal  cavity  at 
the  level  of  the  lower  ribs  and  on  each  side  of  the  vertebral  col- 
umn are  the  kidneys,  the  organs  of  urine  excretion.  Each  kidney 
is  of  the  nature  of  a  tubular  gland  of  a  very  complex  structure, 
anatomically  adapted  to  bring  a  large  amount  of  blood  at  a  high 
pressure  in  close  relation  with  the  excreting  epithelial  cells  which 
line  the  walls  of  the  gland  tubules.  The  tubules  empty  into  a 
pouch-shaped  sac  on  the  inner  edge  of  the  kidney,  the  pelvis 
of  the  kidney,  and  this  is  connected  with  the  urinary  bladder  by 
means  of  a  small  tube,  the  ureter. 

A  brief  review  of  the  essential  parts  of  the  uriniferous  tubule 
and  the  organs  of  micturition  is  necessary  in  order  to  understand 
the  mechanism  of  urine  excretion,  and  the  student  is  advised  to 
consult  his  textbook  of  anatomy  and  histology  for  a  more  Qom- 
prehensive  description  than  is  here  given.  The  uriniferous  tu- 
bules may  be  divided  into  the  excretory  portion  and  the  collect- 
ing portion.     The  tubules  arise  in  the  outer  part  of  the  kidney. 


Fig.    37. Diagram   of   the   uriniferous   tubules    (black),   the   arteries    (red) 

and  the  veins    (blue)    of  the  kidney. 


THE  EXCRETION  OF  URINE.  233 

in  tlxe  region  called  the  cortex,  as  a  body  called  the  Malpighian 
corpuscle.  This  corpuscle  consists  of  the  dilated  end  of  a  tubule 
which  is  invaginated  to  form  a  cup-shaped  vessel,  within  the  cup 
of  which  lies  a  tuft  of  capillaries.  The  capillaries  compose  the 
structure  known  as  the  glomerulus,  and  the  tubular  part  is 
known  as  the  capsule  of  Bowman. 

From  Bowman's  capsule  a  short  neck  leads  into  what  is  known 
as  the  convoluted  tubule,  which  is  a  very  tortuous  vessel  lined 
with  large  epithelial  cells.  This  structure  lies  in  the  cortex 
of  the  kidney  and  is  nourislied  by  the  blood  which  has  already 
been  through  the  glomerular  capillaries.  A  loop  of  the  tubule 
leads  down  into  the  center  or  medullary  portion  of  the  kidney 
and  back  again  to  the  cortex,  where  the  cortex  again  becomes 
very  tortuous,  and  finally  empties,  in  company  with  many  other 
similar  vessels,  into  a  common  collecting  tubule,  which  leads  to 
the  pelvis  of  the  kidney. 

The  Blood  Supply  of  the  Kidney  is  very  large  compared 
with  that  of  the  other  organs  of  the  same  size.  The  renal  arteries 
come  from  the  aorta  and  distribute  their  blood  directly  to  the 
glomeruli  and  the  inner  medullary  portions  of  the  kidney.  The 
vessels  of  the  glomerulus  are  collected  into  an  afferent  vein,  which 
again  breaks  up  into  capillaries  to  supply  the  remaining  struc- 
tures of  the  cortical  portions  of  the  kidney  (Fig.  37). 

The  Nerves  of  the  Kidney. — The  kidney  is  very  richly  sup- 
plied with  vasomotor  nerve  fibers,  which  are  carried  to  it  in  the 
splanchnic  nerves.  Whether  there  are  nerve  fibers  in  either  the 
vagus  or  .splanchnic  nerves  which  have  a  secretory  influence  on 
the  kidney  cells,  is  at  present  an  unsettled  question. 

The  Nature  of  Urine  Excretion. — In  spite  of  repeated  at- 
tempts to  explain  the  nature  of  urine  excretion,  there  remain 
many  steps  in  the  process  which  are  not  fully  understood.  The 
constituents  of  the  urine  are  formed  by  other  organs  than  the 
kidney,  and  are  present  in  the  blood  plasma.  The  function  of 
the  kidney  is  to  remove  these  substances  from  the  blood.  Many 
bodies  are  present  in  the  blood  plasma  which  are  not  found  in  the 
urine,  and  again  some  of  the  urinary  constituents  are  found  in 
far  greater  concentration  in  the  urine  than  in  the  blood  plasma. 


234  HUMAN  PHYSIOLOGY. 

To  explain  these  facts,  Ludwig,  a  famous  physiologist  of  the 
nineteenth  century,  formulated  what  is  known  as  the  mechanical 
theory  of  urine  excretion.  Impressed  by  the  peculiar  relation- 
ship of  Bowman's  capsule  and  the  glomerular  capillaries,  he  con- 
cluded that  the  Malpighian  corpuscle  is  a  filtering  apparatus 
which  separates,  in  dilute  solution,  a  portion  of  all  the  diffusible 
substances  of  the  blood.  The  absence  of  such  diffusible  sub- 
stances as  sugar  in  normal  urine  and  its  presence  in  the  blood  in 
a  relatively  large  amount,  he  believed  to  be  due  to  the  ability  of 
the  epithelium  of  the  tubules  to  reabsorb  these  substances  from 
the  dilute  urine.  Likewise,  the  high  concentration  of  salts  and 
nitrogenous  bodies,  such  as  urea,  he  explained  by  reabsorption 
of  water  through  the  tubules  into  the  blood.  In  support  of  this 
theory  Ludwig  demonstrated  that  the  urine  excretion  varied 
directly  with  the  blood  flow  and  the  blood  pressure  of  the  kid- 
ney. In  other  words,  the  greater  the  supply  of  blood  and  the 
greater  its  pressure,  the  more  rapidly  will  the  watery  solution 
of  the  urine  be  filtered  from  the  blood.  He  was  not  able,  how- 
ever to  bring  any  satisfactory  proof  of  the  reabsorption  of  water 
or  other  substances  by  the  epithelium  of  the  urinary  tubules. 
Indeed,  most  experiments  show  that  this  does  not  occur. 

It  is  impossible  to  explain  all  the  facts  of  urinary  excretion 
by  simple  phj^sical  laws.  For  example,  urea  and  dextrose  are 
both  found  in  the  blood  and  both  obey  the  same  physico-chemical 
laws ;  nevertheless  the  one  is  excreted  in  the  urine  and  the  other 
is  retained  in  the  blood.  Furthermore,  when  certain  pigments 
are  injected  into  the  blood,  they  are  excreted  by  the  kidney  cells, 
but  do  not  appear  in  those  of  other  parts  of  the  body. 

That  an  increase  in  the  pressure  of  blood  in  the  renal  vessels 
has  a  very  marked  accelerating  effect  on  the  excretion  of  urine, 
is  not  necessarily  evidence  that  the  increased  blood  supply  is  the 
cause  of  the  excretion.  That  other  factors  are  concerned  is  demon- 
strated by  the  action  of  drugs  which  cause  an  increase  in  renal  ex- 
cretion. For  example,  digitalis,  a  drug  stimulating  the  circulatory 
apparatus,  causes  a  marked  diuresis  in  cases  of  a  weak  heart 
where  the  pressure  has  been  totally  inadequate  to  maintain  a 
urine  excretion,  but  has  little  or  no  action  on  the  normal  kidney. 


THE  EXCRETION  OP  URINE.  235 

On  the  other  hand,  sodium  sulphate  injected  into  the  blood 
causes  a  diuresis  without  marked  change  in  rate  of  blood  flow 
or  blood  pressure  by  direct  stimulation  of  the  renal  epithelium. 
In  almost  every  case,  moreover,  an  increase  in  the  excretion  of 
urine  is  followed  by  an  increase  in  the  amount  of  oxygen  used 
up  by  the  kidney.  It  is  a  general  law  that  every  increase  in  cell 
activity  is  accompanied  by  an  increase  in  the  amount  of  oxygen 
used  by  the  organ,  and  the  increased  blood  flow  accompanying 
most  forms  of  diuresis  is  readily  explained  on  the  basis  of  the 
physiological  need  of  the  tissue  for  water  and  oxygen.  If  physi- 
cal laws  were  sufficient  to  explain  all  the  phenomena  of  excre- 
tion, there  would  be  no  need  for  oxygen  in  increased  amounts 
during  periods  of  increased  urine  formation.  A  conception  of 
the  actual  amount  of  work  which  the  cells  must  do  to  excrete  the 
urine  may  be  obtained  by  comparing  the  osmotic  pressure  of  the 
urine  with  that  of  the  blood.  The  osmotic  pressure  of  the  blood 
is  only  half  that  of  the  urine,  and  for  each  one  thousand  cubic 
centimeters  excreted,  it  is  sufficient  to  call  for  the  expenditure, 
on  the  part  of  the  renal  cells,  of  a  force  capable  of  lifting  a 
pound  through  one  thousand  feet. 

We  may  conclude  that  the  nature  of  the  excretory  mechanism 
cannot  be  explained  by  the  physico-chemical  laws  as  we  now 
know  them,  i.  e.,  the  phenomena  of  osmosis,  filtration,  absorption, 
etc.,  but  rather  that  it  must  be  due  to  a  vital  action  on  the  part 
of  the  renal  cells.  It  is  this  vital  function  of  the  cells  which 
enables  them  to  remove  one  substance  from  the  blood  and  to  leave 
another  which  is  identically  the  same  so  far  as  physico-chemical 
properties  are  concerned. 

Micturition. — The  urine  discharged  from  the  collecting 
tubules  of  the  kidney  into  the  pelvis,  is  carried  to  the  urinary 
bladder  through  the  ureters  (Fig.  38).  The  muscular  coats  of 
the  ureter  have  a  movement  similar  to  that  of  the  digestive  canal 
and  by  peristaltic  waves  force  the  urine  down  through  the  ureter 
into  the  bladder.  The  urine  thus  collected  by  the  bladder  is 
retained  for  a  time  and  is  at  intervals  ejected  through  the  urethra 
by  the  act  of  micturition.  This  consists  of  strong  contraction  of 
the  bladder  walls,  together  with  the  contraction  of  the  diaphrag- 


236 


HUMAN   PHYSIOLOGY. 


inatic  and  abdominal  muscles,  the  effect  of  which  is  to  reduce  the 
size  of  the  bladder  cavity  and  to  expel  the  urine  with  pressure, 
through  the  urethra. 

The  act  is  under  nervous  control,  the  motor  nerves  being  de- 
rived from  nerve  cells  found  in  the  lumbar  region  of  the  cord. 
The  stimuli  here  produced  co-ordinate  the  muscular  movements 
of  the  act.  The  afferent  or  sensory  stimuli  which  initiate  the 
act  are  excited  by  the  distention  of  the  bladder,  or  by  the  pass- 
age of  a  few  drops  of  urine  into  the  first  portion  of  the  urethra. 
These  stimuli  pass  to  the  center  in  the  cord  and  are  returned  to 


Vena  cavAfS; 


U  reWhro. 
Fig.    SS.^Diagram  of  urinary  system. 


the  muscles  of  the  bladder  also  causing  the  sphincter,  which  closes 
the  bladder  to  be  relaxed.  In  the  voluntary  act  the  motor  nerves 
are  stimulated  by  impulses  from  the  higher  centers. 

The  Function  of  the  Skin. 

The  skin  serves  a  double  function,  that  of  protecting  the  body 
from  the  outside  environment,  and  that  of  excreting  essential 


THE  FUNCTIONS  OF  THE  SKIN.  237 

fluids  from  its  glands.  Contrary  to  general  belief,  the  glands 
of  the  skin  do  not  excrete  the  waste  substances  of  the  body,  or 
at  least  do  so  only  to  a  very  limited  degree.  Their  functions  are : 
to  regulate  the  internal  heat  of  the  body  (sweat  glands)  ;  to  lubri- 
cate its  surface  and  hairs  (sebaceous  glands)  ;  and  to  provide  the 
best  form  of  nourishment  for  the  newborn  animal  (mammary 
glands). 

The  Sweat  Glands. — These  are  simple  coiled  tubular  struct- 
ures, found  practically  everywhere  in  the  cutaneous  tissue  of  the 
body,  being  especially  numerous  in  certain  parts,  as  in  the  palms 
of  the  hands  and  the  soles  of  the  feet.  The  excreting  cells  line 
the  lower  portions  of  the  tubules,  and  are  composed  of  granular, 
columnar  epithelium.  The  glands  are  richly  supplied  with  nerve 
fibers. 

The  amount  of  sweat  given  off  in  a  day  varies  greatly,  since 
it  is  influenced  by  many  things,  as  heat,  moisture,  exercise,  cloth- 
ing, etc.  (see  p.  135).  The  perspiration  of  which  we  are  uncon- 
scious amounts  to  a  considerable  number  of  grams  (700  to  900 
grams)  in  a  day.  Although  it  is  very  difficult  to  obtain  pure 
sweat  unmixed  with  the  secretions  of  the  other  glands  of  the 
skin,  we  know  that  it  consists  for  the  most  part  of  water,  having 
a  specific  gravity  of  about  1.004.  The  salty  taste  is  due  to  inor- 
ganic salts  and  to  the  impurities  which  the  sweat  dissolves  on  the 
surface  of  the  skin.  There  is  only  a  trace  of  urea  and  related 
substances,  and  probably  the  sweat  glands  never  aid  the  kidneys 
in  the  excretion  of  these  bodies. 

The-  most  important  function  of  the  sweat  glands  is  to  control 
the  temperature  of  the  body  by  regulating  the  rate  of  its  heat 
loss.  Dry  air  is  a  poor  conductor  of  heat,  and  to  vaporize  water 
requires  a  large  amount  of  heat.  As  the  water  of  the  sweat  is 
evaporated,  the  body  loses  heat  rapidly.  This  principle  is  practi- 
cally applied  by  the  housewives  of  tropical  countries.  The  water 
is  placed  in  i)orous  pots  and  the  rapid  evaporation  on  the  out- 
side of  the  pot  cools  the  water  within. 

The  secretion  of  sweat,  like  the  secretion  of  saliva,  is  under 
the  control  of  the  central  nervous  system,  as  can  be  demonstrated 
by  electrically  exciting  the  nerves  supplying  the  paw  of  a  cat  or 


238  HUMAN  PHYSIOLOGY. 

dog.  Following  such  stimulation  drops  of  sweat  are  found  on 
the  paw.  The  secretion  is  not  due  to  an  increased  blood  flow,  as 
can  be  shown  by  stimulating  the  nerves  in  a  limb  severed  from 
its  blood  supply,  in  which  case  a  few  drops  of  sweat  will  still 
appear.  A  center  in  the  brain  and  subsidiary  centers  in  the 
spinal  cord  have  been  found  which,  when  stimulated,  produce 
a  secretion  of  sweat. 

Some  drugs  have  the  peculiar  action  of  exciting  the  secretion 
of  sweat,  either  reflexly  through  the  nerve  center  or  by  stimula- 
tion of  the  nerve  endings  about  the  cells  of  the' glands.  To  the 
former  class  belong  such  drugs  as  strychnine  and  picrotoxin,  and 
to  the  latter,  pilocarpin.  Atropin,  on  the  other  hand,  inhibits 
the  secretion  by  paralyzing  the  secretory  nerve  mechanism.  An 
increase  in  the  external  temperature  will  cause  a  secretion  of 
sweat  only  when  the  sensory  and  motor  nerves  of  the  part  are 
both  functional.  To  stimulate  the  sweat  nerves,  heat  therefore 
must  act  reflexly  through  the  sensory  nerves  and  the  centers  of 
the  brain  or  spinal  cord. 

The  Sebaceous  Glands. — Besides  the  sweat  glands  there  are 
numerous  other  glands  in  the  skin.  These  are  associated  with 
the  hairs,  and  are  called  sebaceous  glands.  They  secrete  an  oily 
semiliquid  material  which  affords  protection  to  the  hair  and  the 
skin.  Its  oily  nature  prevents  the  hair  from  becoming  too  brittle, 
and  protects  the  skin  from  moisture. 

The  Secretion  op  Milk. — The  mammary  glands  are  modified 
sebaceous  glands  which  secrete  a  nutrient  fluid,  milk.  The 
glands  are  much  better  developed  in  the  female  than  in  the  male, 
and  are  excited  to  physiological  activity  at  the  birth  of  a  child. 
Human  milk  is  a  white  or  yellowish  fluid,  without  odor  and  with 
a  peculiar  sweet  taste.  It  contains  protein  substances  called 
caseinogen,  lact-albumin,  and  lact-globulin ;  also  a  sugar  called 
lactose  or  milk  sugar,  and  fats  and  inorganic  matter,  as  the  chlo- 
rides of  sodium,  potassium  and  calcium.  Human  milk  is  by  far 
the  best  food  for  the  infant,  and  should  be  replaced  by  other 
food  only  when  absolutely  necessary. 


CHAPTER  XXV. 
THE  NERVOUS  SYSTEM. 

The  General  Functions  and  Structure  of  the  Nervous  System. 
— ^When  a  unicellular  organism,  such  as  the  aijiceba,  is  stimulated 
it  responds  by  a  movement  because  its  protoplasm  possesses 
among  its  other  properties  those  of  excitability,  conductivity  and 
contractility.  In  the  case  of  multicellular  organisms,  some  cells 
are  set  aside  for  the  assimilation  of  food,  others  for  movement, 
others  to  receive  stimuli  from  the  outside,  others  to  compose 
tougher  protective  tissues  on  the  surface,  and  still  others,  in  many 
animals,  to  compose  definite  organs  of  offense.  This  location 
of  specific  functions  in  a  certain  group  of  cells  makes  it  neces- 
sary, for  the  welfare  of  the  organism  as  a  whole,  that  some  means 
of  communication  be  provided  between  the  different  parts  of  the 
animal,  for  otherwise  the  cells  which  are  occupied,  say,  in  ab- 
sorbing food,  would  be  unable  to  move  away  when  some  destruc- 
tive agency  approached  them,  and  indeed  the  moving  (muscle) 
cells  could  never  know  when  they  ought  to  become  active.  In 
some  of  the  lower  organisms  these  messages  are  carried  by  chemi- 
cal substances  present  in  tlie  fluids  that  bathe  the  cells.  These 
belong  to  the  group  of  hormones  which  we  have  already  studied 
in  connection  with  the  ductless  glands  (see  p.  124).  The  re- 
sponses mediated  in  this  way  are,  however,  too  slow  for  the  quick 
adaptation  which  it  is  necessary  that  the  organism  should  un- 
dergo in  its  battle  for  life.  If  it  had  to  depend  on  such  a  mech- 
anism alone,  the  organism  would  already  be  within  the  clutches 
of  its  enemy  before  it  could  make  any  attempt  to  defend  itself. 

Some  more  sensitive  mechanism,  both  for  receiving  and 
for  transmitting  impulses  tlirougliout  the  organism,  becomes  nec- 
essary. This  is  furnished  by  the  nervous  system,  which,  in  its 
simpler  form,  consists  of  a  cell  on  the  surface  of  the  animal  so 
specialized  that  it  responds  to  changes  in  the  environment.    This 

239 


240 


HUMAN   PHYSIOLOGY. 


receptor  cell,  as  it  is  called,  is  prolonged  inside  the  animal  as  a 
fiber,  the  nerve  fiber,  which  passes  to  effector  cells  specialized 
either  as  muscle  fibers  or  gland  cells.  When  a  stimulus  acts  on 
the  receptor  cell  it  therefore  sets  up  a  nerve  impulse  which  causes 
effector  cells  to  become  active,  so  that  the  animal  either  moves 
away  or  prepares  to  defend  itself  by  secreting  some  poisonous 
substance  or  making  some  defensive  movement.  There  are,  how- 
ever, very  few,  even  of  the  lowliest  organising,  which  have  so 
simple  a  nervous  system  as  this,  for  the  nerve  fibers  from  differ- 
ent receptors  usually  join  together  to  form  a  nerve  'plexus  and 
they  do  not  run  directly  to  the  effector  cell,  but  to  another  cell. 


Fig.  39. — Schema  of  simple  reflex  arc;  r,  receptor  in  an  epithelial  mem- 
brane Sa,  afferent  fiber  ;  s,  synapsis  ;  c,  nerve  cell  of  center  ;  e,  efferent  fiber ; 
m,  effector  organ. 


the  central  nerve  cell,  which  is  specialized  as  a  junctional  or  dis- 
tributing center,  and  which  then  transmits  the  impulse  by  a  fiber 
of  its  own  to  the  proper  effector  organs. 

Thus  we  have  the  essential  elements  of  the  so-called  reflex  arc 
(Fig.  39),  that  is,  a'  receptor  connected  with  a  nerve  fiber  called 
afferent  running  to  a  central  nerve  cell  which  is  again  connected 
with  a  nerve  fiber  called  efferent,  which  passes  to  some  effector 
organ.  In  certain  of  the  lower  organisms  these  nerves  and  nerve 
cells  are  continuous  throughout,  but  in  the  higher  animals  the 
fibers  originating  from  each  cell  do  not  actually  join  with  those 


GENERAL   STRUCTURE  OP  THE  NERVOUS  SYSTEM.  241 

of  others,  but  only  come  in  close  contact  with  them.  They  are 
contiguous  but  not  continuous,  and  the  nerve  impulses  pass  from 
one  to  another  by  contact  rather  than  by  transmission  through 
continuous  tissue. 

Every  nerve  cell  gives  off  at  least  one  process  called  the  axon, 
and  it  is  this  which  forms  the  axis  cylinder  of  the  nerve  fiber. 
There  are  usually  other  processes,  but  they  differ  from  the  axon 
in  that  they  branch  freely  and  do  not  run  for  any  distance  from 
the  cell.  They  are  called  dendrites.  The  axon  may  also  occasion- 
ally give  off  a  branch,  often  called  a  collateral,  but  it  is  not  until 
it  has  reached  the  effector  organ  or  some  other  nerve  cell  that 
the  branching  is  pronounced.  It  now  breaks  up  into  a  mass  of 
fine  branches.  When  these  occur  at  a  second  nerve  cell,  they 
closely  encircle  the  cell,  forming  a  basket-like  structure  around 
it.  This  is  called  a  synapsis.  The  nerve  impulse  can  travel  from 
the  fiber  through  its  synapsis  on  to  the  nerve  cell  which  this  sur- 
rounds, but  it  cannot  travel  in  the  opposite  direction.  This  valve- 
like action  at  the  synapsis  explains  why  a  nerve  impulse  travels 
along  a  reflex  arc  in  one  direction  only.  Each  nerve  cell  with 
its  axon  and  dendrites  is  called  a  neurone.  Reflex  arcs  are  there- 
fore composed  of  two  or  more  neurones,  and  the  nervous  system 
is  built  up  of  great  numbers  of  reflex  arcs. 

The  nerve  cells  which  constitute  the  centers  are  usually  col- 
lected in  groups  called  ganglia.  In  the  segmented  invertebrates, 
such  as  the  worms  and  crustaceans,  there  is  one  such  ganglion 
for  each  segment,  each  ganglion  being  connected  with  its  neigh- 
bors by  nerve  fibers,  thus  forming  a  chain  along  the  ventral 
aspect  of  the  animal,  and  also  having  numerous  nerve  fibers  con- 
necting it  with  the  various  receptors  and  effectors  of  the  segment 
(Fig.  40).  At  the  head  end  of  the  animal  several  of  these  gang- 
lia become  fused  together  to  form  a  larger  ganglion,  which  lies 
ju.st  behind  the  gullet  and  from  which  two  fibers  pass  around  the 
gullet  to  unite  in  front  of  it  in  a  large  ganglion,  which  usually 
shows  three  lobes.  These  larger  head  ganglia  receive  the  affer- 
ent nerve  fibers  from  the  adjacent  projicient  sense  organs, 
namely,  the  eyes,  the  ears,  the  organ  of  smell,  and  the  antennje 
or   feelers;    these    being   really    receptors   which   have    become 


242 


HUMAN   PHYSIOLOGY. 


highly  specialized  for  the  purpose  of  receiv- 
ing impressions  from  a  distance.  Many  of 
the  efferent  fibers  which  arise  from  the  cells 
of  the  head  ganglia  go  to  the  muscles  which 
move  the  head  end  of  the  animal,  others,  how- 
ever, do  not  run  directly  to  effectors,  but  they 
run  down  the  nerve  chain  to  make  synaptic 
connection  with  the  cells  of  some  of  the  seg- 
mental ganglia.  This  connection  of  the  cells 
of  the  head  ganglia  with  those  supplying  the 
segments  enables  the  former  to  exercise  a  dom- 
inating influence  oyer  the  activities  of  the  lat- 
ter, the  purpose  being  that  approaching  dan- 
gers may  have  a  greater  influence  in  deter- 
mining the  response  of  the  animal  than  stim- 
uli that  are  merely  local.  When,  for  example- 
some  sight  or  sound  of  an  approaching  enemy 
is  received  by  the  head  ganglia,  these  will 
transmit  impulses  down  the  ganglion  chain 
which  so  influence  the  various  nerve  cells  as  to 
produce,  in  all  of  them,  a  co-ordinated  action 
for  the  purpose  of  getting  the  animal  out  of 
danger.  Even  should  some  local  stimulus  be 
acting  on  one  or  more  of  the  segments,  the 
stimulus  which  is  received  through  the  head 
ganglia  will  obtain  the  upper  hand  and  annul 
or  inhibit  the  local  influence.  The  part  will 
become  subservient  to  the  whole.  This  illus- 
trates the  integration  of  the  nervous  system, 
which,  as  we  pass  to  higher  animals,  we  shall 
find  to  become  more  and  more  developed  and 
intricate. 

So  far,  however,  the  nervous  reaction  is 
purely  of  the  nature  of  a  reflex;  but  in  the 
higher  animals  other  factors,  namely,  memory 
and  volition,  come  to  exercise  a  dominating  in- 
fluence on  the  nature  of  the  response.     The 


Fig.  40.  —  Dia- 
gram of  nervous 
system  of  segment- 
ed invertebrate ;  a, 
sup  racesophageal 
ganglion ;  Tj,  sub- 
cesophageal  gang- 
Jion ;  oe,  oesopha- 
gus or  gullet. 


GENERAL   STRUCTURE  OP   THE  NERVOUS  SYSTEM.  243 

afferent  stimulus  arriving,  let  us  suppose-  at  nerve  cells  controll- 
ing the  movements  of  the  leg,  may  fail  to  cause  a  response  of  the 
corresponding  muscles  because  of  impulses  meanwhile  trans- 
mitted from  higher  memory  centers,  for  the  animal  may  have 
learned  by  experience  that  such  a  movement  as  the  local  stim- 
ulus would  in  itself  call  forth,  is  hurtful  to  its  own  best  inter- 
ests. This  experience  will  have  become  stored  away  as  a  mem- 
ory in  the  higher  (memory)  nerve  centers,  so  that  whenever  the 
local  stimulus  comes  to  be  repeated,  impulses  are  discharged 
from  these  memory  centers  to  the  local  nerve  center  and  the  re- 
flex response  does  not  occur,  or  is  much  modified  in  nature.  For 
storing  away  these  memories  and  for  related  psychological  proc- 
esses of  volition,  etc.,  the  anterior  portions  of  the  nervous  system 
in  the  vertebrates  become  very  highly  developed  so  as  to  consti- 
tute the  hrain,  and  the  simple  chain  of  ganglia  of  the  inverte- 
brates comes  to  be  replaced  by  the  spinal  cord. 

As  we  ascend  the  scale  of  the  vertebrates,  the  brain  becomes 
more  and  more  developed,  until  in  the  higher  mammalia,  such 
as  man,  very  few  reflex  actions  can  occur  independently  of  the 
higher  centers  which  are  located  in  it.  In  other  words,  the  reflex 
arc  now  involves,  not  one  nerve  center,  but  several,  and  of  these 
the  most  important  are  located  in  the  brain. 


CHAPTER  XXVI. 

THE  NERVOUS  SYSTEM  (Cont'd). 

Reflex  Action. 

The  Nerve  Structure  Involved  in  the  Reflexes  of  the  Higher 
Mammals. — In  general,  as  already  mentioned,  these  include  a 
receptor,  an  afferent  fifcer,  a  nerve  center,  an  efferent  fiber  and 
an  effector  organ. 

The  Receptor. — The  receptor  exists  as  one  of  the  sensory 
nerve  terminators  situated  in  the  skin  (extero-ceptors)  or  in  the 
deep  tissues,  such  as  the  joints,  the  muscles  or  the  viscera 
(proprio-ceptors).  Many  receptors  are  highly  specialized  so  as 
to  respond  only  to  one  kind  of  stimulus,  and  each  special  kind 
of  receptor  is  located  where  it  will  be  of  most  use.  Thus,  there 
are  special  receptors  for  sensations  of  heat,  others  for  cold,  others 
for  touch,  others  for  pain.  The  pam  receptors  are  distributed 
more  or  less  uniformly  over  the  body.  They  are  present  in  the 
deeper  structures,  such  as  the  teeth,  the  joints  and  the  serous 
coverings  of  the  viscera.  Sometimes,  as  on  the  cornea  and  in  the 
pulp  of  the  teeth,  they  are  the  only  kind  of  receptor  present. 
The  touch  receptors  are  collected  in  small  areas  called  ''touch 
spots, ' '  which  are  much  more  numerous  on  the  tip  of  the  tongue, 
the  lips,  or  the  tips  of  the  fingers  than  on  the  skin  of  the  legs, 
the  arms  or  the  back  of  the  trunk.  The  frequency  of  touch  spots 
on  the  tip  of  the  tongue  makes  a  foreign  body  in  the  mouth  ap- 
pear to  be  larger  than  when  we  feel  it  with  the  fingers.  The 
touch  spots  on  the  finger  tips  may  acquire  great  acuity  of  per- 
ception by  education,  as  in  the  case  of  a  blind  person,  who  has 
to  use  his  fingers  for  reading.  The  remarkable  irregularity  of 
distribution  of  touch  spots  may  be  very  beautifully  shown  by 
finding  out  how  far  apart  the  points  of  a  pair  of  calipers  must 
be  from  each  other  in  order  to  be  distinguished  as  separate. 
This  distance  is  not  more  than  3  mm.  for  the  tips  of  the  fingers, 

244 


Fig.  41. — The  simplest  reflex  arc  in  the  spinal  cord.  (After  Kolliker.) 
The  afferent  fiber  in  the  posterior  root  (in  black)  gives  off  collaterals,  which 
end  by  synapses  around  the  cells  of  the  anterior  horn  (in  red),  the  axons 
of  which  form  the  efferent  fibers  of  the  anterior  roots.  (From  Howell's 
Physiolog-y. ) 


REFLEX  ACTION. 


245 


but  it  is  over  60  mm.  for  the  skin  of  the  back  of  the  neck.  The 
temperature  receptors  are  still  more  definitely  located  in  areas, 
some  being  specialized  for  heat  and  others  for  cold.  These  so- 
called  heat  and  cold  spots  are  most  frequent  on  the  portions  of 
the  body  that  are  covered  by  clothing,  for  example,  the  skin  of 
the  thorax,  than  on  those  that  are  exposed,  for  example,  the  face. 
They  are  fairly  frequent  on  the  skin  of  the  dorsum  of  the  hand, 
where  their  existence  can  be  very  easily  demonstrated  by  slowly 
drawing  a  pencil  gently  over  the  skin.  At  certain  places  the 
point  of  the  pencil  feels  hot,  at  others  cold,  and  in  others  it 
causes  no  temperature  sensation  whatsoever. 

All  varieties  of  receptors  are  present  on  the  skin  of  the  hand, 
but  in  certain  diseases  of  the  nerves  or  spinal  cord,  one  kind  of 
receptor  may  become  inactive,  thus  causing,  when  the  absent  sen- 
sation is  that  of  pain,  the  condition  called  analgesia,  which  must 
be  distinguished  from  that  of  anesthesia,  when  all  sensations  are 
paralyzed.  In  analgesia  a  pin  prick  causes  only  a  sensation  of 
touch.  When  the  nerves  of  the  arm  are  cut  and  the  cut  ends 
then  sutured  together  so  that  the  nerve  fibers  regenerate,  the  skin 
sensations  do  not  all  return  at  the  same  time.  Those  of  pain  and 
of  extreme  degrees  of  heat  and  cold  return  in  from  six  to  twenty- 
six  weeks,  whereas  those  of  touch  and  the  finer  degrees  of  tem- 
perature do  not  return  until  after  one  or  two  years.  The  power 
of  localizing  the  point  of  application  of  the  stimulus  is  also  late 
in  returning;  thus,  if  we  touch  the  finger  of  such  a  person  and 
ask  him  to  tell  us  where,  he  may  indicate  some  spot  that  is  quite 
a  distance  away  from  the  one  actually  touched.  Certain  drugs, 
such  as  cocaine,  have  the  power,  when  applied  locally,  of  ren- 
dering all  the  receptors  insensitive. 

The  Afp'erent  Fiber. — Another  name  for  this  is  the  sensory 
nerve,  because  it  carries  the  sensations  received  by  the  receptors 
up  to  the  nerve  center.  All  afferent  fibers  enter  the  spinal  cord  by 
the  posterior  nerve  roots,  on  each  of  which,  it  will  be  remem- 
bered, is  situated  a  ganglion,  the  posterior  root  ganglion.  The 
cells  of  this  ganglion  are  connected  with  the  afferent  fibers  by 
a  short  branch  running  at  right  angles  to  the  latter  (Fig.  41). 
The  function  of  the  cells  is  to  maintain  the  nutrition  of  the  affer- 


246  HUMAN  PHYSIOLOGY. 

ent  fibers,  for  if  these  be  divided  before  they  reach  the  ganglion, 
the  peripheral  or  far  away  end  undergoes  degeneration,  whereas 
if  the  cut  be  made  between  the  ganglion  and  the  cord,  degenera- 
tion occurs  central-wards,  that  is,  towards  and  into  the  cord.  This 
degeneration  always  occurs  in  the  portion  of  the  nerve  fiber  which 
has  been  disconnected  from  the  nerve  cell.  It  therefore  furn- 
ishes us  with  a  ready  method  for  finding  out  whether  the  fiber 
is  running  towards  or  away  from  the  brain.  In  the  former  case, 
the  fiber  is  said  to  be  ascending,  and  it  degenerates  above  the 
section;  in  the  latter  case,  it  is  descending  and  it  degenerates 
below  the  section.  Since  degenerated  nerve  fibers  give  charac- 
teristic staining  reactions,  we  are  thus  furnished  with  a  means 
of  fi.nding  out  what  becomes  of  the  afferent  fibers  after  they 
enter  the  cord. 

To  further  trace  the  course  and  connections  of  the  afferent  fib- 
ers in  the  cord,  we  must  therefore  cut  the  posterior  roots  between 
the  ganglion  and  spinal  cord  and  after  a  few  weeks  kill  the 
animal  and  make  microscopic  examination  of  the  cord,  stained 
in  special  ways.  If  we  take  a  series  of  such  sections  above  the 
level  at  which  the  posterior  roots  have  been  cut,  we  shall  find 
that  opposite  the  point  of  entry  of  the  cut  root,  the  degenerated 
fibers  occupy  an  area  near  the  tip  of  the  posterior  horn  of  grey 
matt-er.  As  we  examine  sections  taken  higher  and  higher  up, 
the  degenerated  area  will  be  found  to  shift  gradually  towards 
the  median  fissure,  occupying,  first  of  all,  the  so-called  postero- 
lateral column,  and  later  the  postero-median  (Fig.  42).  "When 
we  get  to  the  medulla  oblongata  or  ''bulb,"  the  degenerated 
areas  disappear  because  the  fibers  have  terminated  by  forming 
synapses  around  the  cells  of  the  two  large  ganglia  which  form 
the  bulgings  seen  on  the  posterior  aspect  of  this  structure.  The 
fresh  relay  of  nerve  fibers  do  not  degenerate  after  section  of  the 
posterior  roots,  but  by  other  means  of  investigation  they  have 
been  found  to  become  collected  into  a  bundle  called  the  fillet, 
which  crosses,  or  decussates,  to  the  other  side  of  the  medulla  and 
runs  up  through  the  pons  varolii  and  crura  cerebri,  some  of  the 
fibers  ending  near  the  optic  thalamus,  whilst  others  run  on  to 
the  grey  matter  of  the  motor  areas  of  the  cerebrum. 


REFLEX  ACTION. 


247 


The  posterior  root  fiber,  shortly  after  entering  the  cord,  gives  off 
a  branch  at  right  angles  (called  a  collateral),  or  in  its  course  up 
the  cord  it  may  give  off  several  collaterals,  their  destination 
being  the  grey  matter  of  the  cord,  in  which  they  terminate  by 


ventfcvl 
pd 


Fig.  42. — Diagram  of  section  of  spinal  cord,  showing  tracts.  (After  K61- 
liker)  ;  g,  posterior  median,  and  b,  postero-lateral  columns ;  p.c,  crossed 
pyramidal,  and  p.d.,  direct  pyramidal  tracts ;  /,  cerebellar  tract.  (After 
Howell.) 


synapses  around  nerve  cells.  Certain  of  these  may  be  cells  of 
the  anterior  horn.  These  cells  give  rise  to  the  efferent  fibers, 
which  leave  the  spinal  cord  by  the  anterior  or  motor  roots  (see 
Fig.  41).  Other  collaterals  run  to  intermediary  cells,  which 
then  communicate  with  the  anterior  horn  cells  (Fig.  43). 

The  Nerve  Center  and  Intermediary  Neurones. — When 
the  entering  nerve  impulse  travels  by  a  collateral  to  an  anterior 
horn  cell,  wc  have  the  simplest  type  of  reflex  action,  namely,  one 
involving  a  receptor,  a  sensory  nerve  fiber,  the  posterior  root,  a 
collateral,  the  anterior  horn  cell,  the  anterior  root,  a  motor  nerve 
fiber  and  an  effector  organ.     But  such  a  simple  reflex  seldom 


248  HUMAN   PHYSIOLOGY. 

occurs  in  the  higher  animals.  The  afferent  impulse  when  it  en- 
ters the  cord  is  more  likely  to  travel  up  the  posterior  columns 
and  then,  as  already  outlined,  to  the  cereBrum,  where  it  is  trans- 
mitted to  the  large  pyramidal  nerve  cells  of  the  grey  matter. 

From  the  pyramidal  cells  spring  the  fibers  of  the  pyramidal 
tracts,  which,  as  they  pass  downward  through  the  white  matter  of 
the  cerebrum,  crowd  closer  and  closer  together  until,  by  the  time 
the  basal  ganglia  are  reached  (optic  thalamus  on  the  inside, 
and  corpus  striatum  on  the  outside),  they  form  a  narrow  bun- 
dle which  occupies  the  middle  portion  of  the  strip  of  white  mat- 
ter, which  lies  between  these  ganglia.  This  white  matter  is 
called  the  internal  capsule  (Fig.  46),  and  it  is  of  very  great 
clinical  interest  because,  being  in  the  neighborhood  of  a  large 
artery  (branch  of  middle  cerebral),  which  sometimes  bursts  in 
elderly  people,  it  is  apt  to  become  torn  up  by  extravasated 
blood,  thus  destroying  the  pyramidal  fibers  and  causing  paraly- 
sis. This  is  what  occurs  in  apoplexy.  Below  the  internal  capsule 
the  fibers  run  into  the  crura  cerebri,  then  into  the  pons,  thence 
into  the  medulla  oblongata,  in  the  front  of  which  they  form  a  dis- 
tinct bulging  called  the  pyramid;  hence  their  name  pyramidal 
fibers  (see  Fig.  45). 

.  Jn  the  lower  portion  of  the  medulla,  a  most  interesting  thing 
occurs,  namely,  three-fourths  of  the  fibers  cross  to  the  oppo- 
site side,  thus  constituting  the  decussation  of  the  pyramids 
(Fig.  44).  These  crossed  fibers  run  down  in  the  lateral  columns 
of  the  spinal  cord  as  the  crossed  pyramidal  tracts.  The  pyra- 
midal fibers  which  do  not  cross  in  the  medulla  form  the  direct 
pyramidal  tracts  of  the  cord,  and  they  gradually  cross  in  the 
cord  itself.  The  pyramidal  fibers  end  by  synapsis  around  the 
cells  of  the  anterior  horn,  so  that  all  fibers  from  the  cerebrum 
ultimately  cross  to  the  opposite  side  before  they  reach  the  anterior 
horn  cells,  for. which  reason  it  happens  that  a  lesion  involving 
the  pyramidal  tract  anywhere  above  the  decussation,  such  as  a 
haemorrhage  in  the  internal  capsule  above  referred  to,  always 
causes  paralysis  of  the  opposite  side  of  the  body  (hemiplegia). 

These  facts  regarding  the  course  of  the  pyramidal  fibers  have 
been  ascertained  by  microscopic  examination  of  sections  from 


which    an    intermediary 


Fig.    43. — Reflex    arc    through    the    spinal    cord, 
neurone   (in  blue)   exists  between  the  afferent  and  efferent  neurones.      (From 
Howell's    Physiology.) 


Fig.  44. — Course  of  the  pyramidal  fibers  from  the  cerebral  cortex  to  thfi 
spinal  cord  :  1,  fibers  to  nuclei  of  cranial  nerves  ;  3,  fibers  which  do  not  cross 
in  the  medulla  (direct  pyramidal  tract)  ;  //  and  .7,  fibers  which  cross  in  medulla 
(crossed  pyramidal  tract).      (After  Howell.) 


REFLEX  ACTION.  249 

various  levels  of  the  spinal  cord  some  time  after  destruction  of 
the  Rolandic  area  of  the  cerebrum  (see  p.  270).  The  pyramidal 
fibers  are  degenerated  and  they  occupy  the  areas  indicated 
in  Fig.  42.  Since  the  degeneration  occurs  below  the  destruction, 
it  is  called  descending  degeneration,  in  contradistinction  to  as- 
cending degeneration,  which  we  saw  to  follow  section  of  the 
posterior  roots  between  their  ganglia  and  the  cord  (see  p.  246). 

To  sum  up,  the  sensory  impulse  on  entering  the  spinal  cord 
by  the  posterior  root,  by  traversing  a  collateral,  may  take  the 
shortest  possible  pathway  to  the  efferent  nerve  cell  of  the  an- 
terior horn,  or  it  may  avoid  this  and  travel  up  the  posterior 
columns  of  the  cord  to  the  medulla,  thence  by  the  fillet  to  the 
cerebral  cortex  of  the  opposite  side,  and  thence  down  the  pyra- 
midal tracts  to  the  anterior  horn  cells.  In  this  long  cerebral 
route  there  are  at  least  three  places  where  the  impulse  must  pass 
by  means  of  a  synapsis  from  nerve  fibers  on  to  nerve  cells,  and 
then  along  the  nerve  fibers  arising  from  these.  These  three 
places  are:  (1)  in  the  medulla,  (2)  in  the  cerebral  cortex,  (3) 
in  the  anterior  horn. 

This  long  cerebral  route,  as  it  is  called,  is  by  no  means  the 
only  one  along  which  afferent  impulses  may  travel  to  the  brain. 
Some  may  be  carried  by  collaterals  to  certain  cells  of  the  grey 
matter  of  the  cord,  and  from  these  cells  fibers  may  run  up  the 
cord  to  the  cerebellum  or  lesser  brain.  These  cerehellar  tracts 
are  located  in  the  lateral  columns  of  the  cord  outside  the  crossed 
pyramidal  tracts  (see  Fig.  42).  They  do  not  degenerate  when 
the  posterior  roots  are  cut,  but  do  so  after  section  of  the  cord 
itself  (this  distinguishing  them  from  the  fibers  in  the  posterior 
columns).  The  impulses  which  they  transmit  to  the  cerebellum 
have  to  do  with  certain  subconscious  sensations  concerned  in 
the  maintenance  of  the  tone  of  the  muscles.  There  are  also 
certain  pathways  in  the  white  matter  of  the  cord  which  trans- 
mit descending  impulses  from  the  cerebellum. 

The  main  bundles  of  ascending  and  descending  fibers  in  the 
spinal  cord  are  charted  in  Fig.  42,  which  should  be  carefully 
studied. 

TiiE   Efferent   Fiber,   or   Neurone. — As  already  explained 


250  HUMAN   PHYSIOLOGY. 

the  cell  of  this  neurone  is  located  in  the  anterior  horn  of  grey 
matter  of  the  cord.  These  anterior  horn  cells  are  distinguished 
from  the  other  nerve  cells  of  the  grey  matter  by  their  large  size 
and  angular  shape,  and  they  become  greatly  increased  in  num- 
ber in  the  portions  of  the  cord  from  which  the  nerves  going  to 
the  extremities  originate.  The  fibers  springing  from  them  pass 
out  in  the  anterior  roots.  If  the  cells  are  destroyed  or  the  an- 
terior roots  cut,  degeneration  occurs  below  the  lesion,  and  para- 
lysis of  the  effector  organs  (muscles)  to  which  they  run  results, 
but  this  paralysis  is  very  slight  in  degree  unless  the  lesion  af- 
fects several  roots,  or  the  cells  of  several  adjacent  levels  of  the 
cord.  The  reason  for  this  is  that  the  nerve  cells  of  one  level  of 
the  cord  only  partially  supply  a  given  muscle  or  group  of  mus- 
cles with  nerve  fibers,  thus  showing  that  even  the  small  muscles 
receive  their  nerve  fibers  from  several  adjacent  levels  of 
the  cord.  The  anterior  horn  cells  sometimes  become  destroyed 
by  disease,  namely,  in  infantile  paralysis  (poliomyelitis  anter- 
ior).   The  resulting  paralysis  is  never  recovered  from. 

Types  of  Reflexes. — Having  traced  the  paths  through  which 
reflexes  occur  in  the  higher  animals,  we  may  now  proceed  to 
consider  certain  typical  forms  of  reflex  action  and  the  condi- 
tions which  may  cause  them  to  become  altered.  We  must  flrst 
of  all  conflne  our  attention  to  the  characteristic  reflexes  of  the 
so-called  spinal  animal,  for  it  is  only  after  we  have  done  so  that 
it  will  be  possible  for  us  to  determine  what  influence  the  brain 
has  in  modifying  the  spinal  reflexes.  The  spinal  animal  (dog, 
for  example)  is  prepared  by  cutting  across  the  spinal  cord  some- 
where below  the  origin  of  the  phrenic  nerves.  After  the  imme- 
diate effects  of  the  operation  have  been  recoverd  from,  the 
regions  of  the  animal's  body  lying  below  the  level  of  the  sec- 
tion of  the  cord,  suffer  from  a  condition  called  spinal  shock. 
All  reflex  movements  are  absent,  the  sphincters  are  paralyzed  so 
that  incontinence  of  urine  and  fasces  exists,  and  various  "tro- 
phic" or  nutritive  changes  occur  in  the  skin  (abscesses  form, 
hair  falls  out,  etc.).  After  some  time,  the  length  of  which  de- 
pends on  the  position  of  the  animal  in  the  animal  scale,  the 
sphincters  regain  their  tone  and  the  reflexes  gradually  reappear 


REFLEX  ACTION.  251 

in  the  paralyzed  region,  the  first  to  do  so  being  the  protective 
reflexes,  of  which  the  flexion  reflex  is  the  type. 

The  flexion  reflex  is  elicited  by  any  stimulus  which  would  cause 
pain  in  an  animal  capable  of  feeling.  Such  stimuli  are  called 
nocuous  and  the  reflex  response  is  always  of  such  a  nature — 
usually  flexion — as  to  cause  the  injured  part  to  be  removed 
from  further  damage.  The  return  of  the  flexion  reflex  is  soon 
followed  by  that  of  the  knee  jerk,  which  is  elicited  by  tapping 
the  patellar  tendon  after  putting  it  on  the  stretch  by  passively 
bending  the  knee  joint.  Somewhat  later  in  many  animals  (e.g., 
dog)  the  scratch  reflex  appears,  so-called  because  it  consists  of 
a  scratching  movement  of  the  hind  leg  in  response  to  mechanical 
irritation  of  the  flank  of  the  animal.  It  is  a  reflex  of  very  great 
interest  because  it  illustrates  to  what  a  remarkable  degree  the 
spinal  cord,  unaided  by  the  brain,  is  capable  of  bringing  about 
complicated  and  purposeful  co-ordinated  movement.  Later  still, 
in  the  lower  animals,  practically  all  the  reflex  movements  which 
a  normal  animal  exhibits  may  reappear. 

"When  the  cord  becomes  severed  in  man,  as  by  spinal  fracture, 
spinal  shock  is  extremely  profound,  and  in  order  to  keep  the 
patient  alive  great  care  must  be  taken,  on  account  of  the  incon- 
tinence of  urine,  to  prevent  infection  of  the  bladder  and  kidneys 
and  to  protect  the  skin  from  ulceration  (bed  sores).  Even  in 
such  cases,  however,  many  of  the  reflexes  recover  in  the  para- 
lyzed regions,  but  the  recovery  is  slow  and  the  limbs  invariably 
atrophy.  It  is  particularly  important  to  note  that  the  time  of  re- 
appearance of  the  reflexes  bears  a  relationship  to  the  degree  of 
development  of  the  cerebral  hemispheres,  thus  rendering  it  evi- 
dent that  spinal  shock  is  due  to  a  break  in  the  nerve  paths  which 
lead  to  and  from  the  brain.  The  higher  the  animal,  the  more 
frequently  do  all  reflex  acts  involve  a  cerebral  path  instead  of 
taking  the  short  cuts  available  through  the  collaterals  (see  p. 
243).  From  usage,  as  it  were,  the  cerebral  paths  become  so  well 
develoxjed  that  when  they  are  suddenly  severed,  the  reflex  action 
becomes  impossible  until  the  entering  afferent  impulse  has 
learned  to  use  the  hitherto  unused  short  cuts  available  through 
collaterals.     When  completely  recovered  from  spinal  shock,  an 


252  HUMAN  PHYSIOLOGY. 

animal,  say  a  dog,  in  so  far  as  voluntary  movement  is  con- 
cerned, is  entirely  paralyzed  in  all  portions  of  the  body  below 
the  level  of  the  section  of  the  cord.  It  cannot  voluntarily  move 
the  affected  parts,  it  cannot  walk,  it  feels  no  pain  or  any  other 
sensation  below  the  lesion,  and  yet  when  appropriately  stimu- 
lated, the  paralyzed  limbs  may  reflexly  undergo  various,  often 
very  complicated  movements. 

The  Essential  Characteristics  of  Reflex  Action. — As  studied 
on  a  perfectly  recovered  spinal  dog  these  are  as  follows : 

1.  For  a  certain  interval  after  applying  the  stimulus  there 
is  no  response,  the  duration  of  this  "latent  period"  depending 
partly  on  the  nature  of  the  reflex  (short  in  the  protective  re- 
flexes, long  in  the  scratch  reflex)  and  partly  on  the  strength  of 
the  stimulus. 

2.  The  response  may  persist  for  some  time  after  the  stimulus 
is  removed  (after  response). 

3.  The  degree  of  the  response  is  roughly  proportional  to  the 
strength  of  the  stimulus,  except  in  certain  of  the  protective  re- 
flexes, such  as  the  conjunctival,  which  consists  in  the  closing  of 
the  eyelids  when  anything  touches  the  eye. 

4.  The  response  is  often  rhythmical  in  character,  even  though 
the  stimulus  be  continuously  applied.  This  is  well  seen  in  the 
scratch  reflex, 

5.  ■  There  are  certain  ways,  apart  from  an  alteration  in  the 
stimulus,  by  which  we  may  cause  a  reflex  movement  to  become 
increased  or  decreased.  Thus,  taking  the  flexion  reflex  as  an 
example,  the  flexion  may  be  (diminished:  (!)  By  stimulating 
some  other  reflex  movement  which  involves  the  same  muscles, 
but  which  is  antagonistic  to  flexion,  e.g.,  by  stimulating  the 
opposite  limb  and  causing  the  so-called  crossed  extension  reflex. 
(2)  By  causing  strong  afferent  impulses  to  pass  through  other 
levels  of  the  spinal  cord,  e.  g.,  pinching  the  tail.  A  similar 
'interference"  is  well  illustrated  in  the  case  of  man  by  stimulat- 
ing the  fifth  nerve  by  firm  pressure  on  the  upper  lip  at  a  time 
when  there  is  an  inclination  to  sneeze.  The  sneezing,  which  is 
a  reflex  due  to  irritation  of  the  mucosa  of  the  nose,  can  usually 
be  prevented.     Expressing  this  phenomenon  of  reflex  interfer- 


REFLEX  ACTION.  253 

ence  in  popular  language,  we  may  say  that  when  the  attention 
of  a  segment  of  the  cord,  or  its  extension  in  the  brain  is  taken 
up  by  some  other  stimulus,  a  reflex  already  in  action,  or  about 
to  act,  is  depressed.  Pain,  such  for  example  as  toothache,  may 
likewise  be  lessened  by  applying  counter-irritation  such  as  a 
blister  to  some  neighboring  skin  area.  (3)  By  means  of  certain 
drugs  known  as  anesthetics,  which  depress  the  excitability  of  the 
nerve  cells.     (4)  By  fatigue. 

The  reflex  movement  may  be  increased:  (1)  by  applying  a 
second  stimulus  to  some  other  area  of  skin  of  the  same  hind  leg 
or  by  applying  electrical  stimulation  to  the  central  end  of  one 
of  its  sensory  nerves;  (2)  by  raising  the  excitability  of  the 
nerve  centers  by  certain  drugs,  such  as  strychnine;  (3)  by  first 
of  all  causing  the  movement  to  disappear,  though  the  stimulation 
causing  it  is  maintained,  by  exciting  some  other  part  of  the 
body  (see  above).  When  the  reflex  reappears  it  is  much  more 
pronounced  than  formerly. 

Muscular  Tone  and  Reciprocal  Action  of  Muscles. — ^Having 
learned  some  of  the  general  characteristics  of  the  reflex  move- 
ments, we  may  now  proceed  to  inquire  into  the  method  by  which 
the  spinal  cord  is  enabled,  by  itself,  so  to  direct  the  afferent  im- 
pulses which  enter  it,  that  the  nerve  cells  of  the  anterior  horn 
discharge  suitable  impulses  to  bring  about  such  complicated 
movements  as  have  just  been  described.  When  a  motor  nerve 
or  an  anterior  spinal  root  is  stimulated,  the  muscles  which  con- 
tract are  not  grouped  in  such  a  way  as  to  cause  any  purposeful 
or  co-ordinated  movement.  Contractors,  extensors,  adductors 
and  abductors  are  quite  likely  all  to  contract  at  once  and  by 
thus  opposing  one  another  to  effect  no  definite  movement.  When 
such  stimulation  is  extensive  (e.g.,  involves  a  considerable  num- 
ber of  motor  fibers),  it  is  common  to  find  that  the  extensor 
muscles  predominate  over  the  others,  so  that  the  limb  becomes 
extended.  Such  is  the  case  when  some  poisonous  substance 
causes  irritation  of  the  nerve  centers  in  the  spinal  cord. 

To  cause  a  co-ordinated  movement  it  is  necessary  that  one 
group  of  muscles  should  become  relaxed  whilst  their  antagonistic 
group  is  undergoing  contraction.    Now,  it  might  at  first  sight  be 


254  HUMAN   PHYSIOLOGY. 

imagined  that  this  relaxation  is  merely  a  passive  act,  that  is  to 
say,  that  the  Tincontracting  group  of  muscles  do  nothing  more 
than  remain  quiescent  and  permit  themselves  to  be  stretched. 
But  such  is  not  the  case;  on  the  contrary,  they  become  actively 
extended.  This  they  are  enabled  to  do  because  of  the  fact  that, 
even  when  apparently  relaxed,  a  muscle  is  really  not  so,  but 
exists  in  a  condition  called  tone,  that  is,  in  a  slightly  contracted 
state.  This  tone  becomes  greatly  diminished  during  sleep,  and  it 
can  be  caused  almost  to  disappear  by  deep  anesthesia.  It  is  for 
this  purpose,  as  well  as  to  abolish  pain,  that  anesthetics  are 
administered  before  attempting  to  reduce  a  dislocation. 

Tone  is  maintained  by  the  nerve  cells  of  the  anterior  horn  of 
the  spinal  cord.  When  therefore  an  afferent  impulse  brings 
about  flexion  at  the  knee  joint,  it  does  so  by  exercising  two 
diametrically  opposite  influences  on  the  anterior  horn  cells:  it 
stimulates  those  which  preside  over  the  flexor  muscles  and  de- 
presses the  tonic  influence  of  those  supplying  the  extensors. 
This  tone-depressing  action  recalls  the  inhibitory  influence  which 
the  vagus  nerve  exercises  over  the  heart  beat  (see  p.  185),  and 
since  it  always  occurs  along  with  a  contraction  of  antagonistic 
muscles  it  is  called  reciprocal  inliibition.  Certain  poisons,  par- 
ticularly strychnine  and  tetanus  toxin,  cause  this  reciprocal 
action  to  break  down  so  that  all  the  muscles  around  a  joint  con- 
tract at  the  same  time  and  produce  an  extension.  Tetanus 
toxin  is  the  poison  produced  by  the  tetanus  bacillus,  and  its 
interference  with  the  reciprocal  inhibition  of  the  muscles  of  the 
lower  jaw  causes  lockjaw. 

Symptoms  Due  to  Lesions  Affecting  the  Reflexes. — From 
what  we  have  learned  regarding  the  functions  of  the  spinal 
cord,  it  is  easy  for  us  to  explain  the  following  symptoms  and 
conditions  resulting  from  pathological  destruction  or  stimula- 
tion of  various  parts  of  it : 

1.  In  destruction  of  the  continuity  of  the  afferent  or  efferent 
fibers  of  the  reflex  arc,  the  reflexes  are  absent.  This  occurs  in 
chronic  in^ammation  of  the  nerves  (neuritis)  and  in  the  disease 
called  locomotor  ataxia,  in  which  the  lesion  consists  of  a  de- 
structive pathological  process  involving  the  posterior  columns 


REFLEX  ACTION.  255 

of  the  spinal  cord.  One  of  the  first  symptoms  of  locomotor 
ataxia  is  absence  of  the  knee  jerk,  which,  it  will  be  remembered, 
is  elicited  by  tapping  the  patellar  tendon  after  putting  it  pas- 
sively on  the  stretch,  either  by  sitting  with  the  feet  swinging  on 
the  edge  of  a  table,  or  by  crossing  one  knee  over  the  other.  Pains, 
called  crises,  are  also  usual  in  various  parts  of  the  body.  Later 
symptoms  are  inability  to  stand  without  falling  when  the  eyes 
are  shut,  inco-ordinated  walking,  in  which  the  foot  is  lifted  too 
high  and  is  brought  down  to  the  ground  again  too  violently,  loss 
of  sensation  of  the  skin  of  the  foot  and  leg,  and  changes  in  the 
pupillary  reflexes  of  the  eye  (see  p.  284).  The  joints  also  be- 
come swollen  and  the  articular  surfaces  roughened  so  that  a 
grating  sensation  is  experienced  when  the  joint  is  bent  (Char- 
cot's joint).  The  condition  gradually  gets  worse,  so  that  the 
patient  becomes  bedridden.  Death  is  usually  due  to  complica- 
tions. ;     I       '    '■■i|^||^>^ 

2.  Destruction  of  the  anterior  horn  cells  not  only  causes 
absence  of  reflex  action,  but  is  followed  by  marked  atrophy  of 
the  affected  muscles.  It  has  been  supposed  that  this  points  to 
a  so-called  trophic  influence  of  these  nerve  cells,  that  is  to  say, 
a  power  of  influencing  nutrition.  Such  changes  occur  in  infan- 
tile paralysis  (poliomyelitis  anterior). 

3.  Stimulation  of  the  above  fibers  may  cause  exaggeration  of 
the  reflexes,  as  in  the  earlier  irritative  stages  of  neuritis,  in 
tumors  pressing  on  the  nerve  roots,  or  when  the  membranes  of 
the  cord  become  inflamed,  as  in  meningitis. 

4.  Removal  of  impulses  coming  from  the  cerebrum  by  way  of 
the  pyramidal  tracts  causes  exaggerated  reflexes.  Such  occur 
in  paralysis  of  both  sides  of  the  body  in  paraplegia,  and  on  the 
paralyzed  side  in  hemiplegia. 

In  a  paraplegic  patient  the  weakest  stimulus  applied  to  the 
skin  of  the  paralyzed  portion  of  the  body  will  call  forth  a  wide- 
spread and  much  exaggerated  reflex  contraction. 


CHAPTER  XXVII. 

THE  NERVOUS  SYSTEM  (Cont'd). 

The  Brain  Stem  and  the  Cranial  Nerves. 

The  Brain  Stem. —  The  medulla,  the  pons  Varolii,  and  the  mid- 
brain (Figs.  45  and  46),  compose  the  brain  stem,  which  is  really 
an  upward  extension  of  the  grey  matter,  and  of  certain  of  the 
columns  of  the  spinal  cord,  into  the  base  of  the  brain  with  special 
nerve  centers  and  especially  large  bundles  of  inter-connecting 
nerve  fibers  superadded.  It  is  because  of  the  crossing  in  various 
directions  of  these  bundles  of  fibers  that  the  structure  of  the 
medulla,  pons  and  mesencephalon  is  so  difficult  to  understand. 
The  grey  matter,  as  in  the  spinal  cord,  lies  deep  and  the  fibers  are 
superficial.  Of  the  latter,  the  pyramids  and  fillet,  already  de- 
scribed, are  the  most  important,  and  their  direction  is  longi- 
tudinal. The  most  prominent  of  the  connecting  or  commisural 
nerve  bundles  are  the  upper,  middle  and  lower  peduncles  of  the 
cerebellum,,  or  small  brain,  which,  it  will  be  remembered,  lies 
over  and  at  the  side  of  the  pons  varolii  and  midbrain.  The 
lower  peduncles  spring  from  the  medulla  and  connect  the  spinal 
cord  with  the  cerebellum.  They  form  the  lower  edges  of  the 
fourth  ventricle.  The  middle  peduncles  enter  the  sides  of  the 
pons,  in  which  they  cross  at  right  angles  with  the  pyramidal 
fibers  (p.  248).  They  connect  the  cerebellum  of  one  side  with 
the  cerebrum  of  the  opposite  side.  The  superior  peduncles  join 
the  encephalon  just  under  the  posterior  corpora  quadrigemina, 
and  the  fibers  composing  them  decussate  to  the  other  side  to  be- 
come connected  with  certain  of  the  so-called  basal  ganglia. 

The  dasal  ganglia  are  the  optic  thalamus  and  the  corpora  stri- 
ata, two  large  collections  of  nerve  cells  protruding  into  the  third 
and  lateral  ventricles  of  the  brain  and  having  the  internal  capsule 
between  them  (see  p.  248).  The  nerve  cells  composing  these 
ganglia  receive  impulses  from  nerve  fibers  arriving  at  them  both 

256 


THE   BRAIN    STEM. 


257 


from  below  (coming  from  the  spinal  cord)  or  from  above  (com- 
ing from  the  cerebrum).  They  then  transmit  these  impulses 
along  their  own  nerve  fibers,  which  ma}^  run  to  various  other 


Fig.  45. — Under  aspect  of  human  brain.  In  the  center  line  from  below 
upward.s  are  seen  a  section  of  the  upper  end  of  the  spinal  cord,  and  the 
medulla  oblongata  (»i),  with  certain  of  the  cranial  nerves*  (as  numbered). 
In  front  of  this  is  the  pons  (}}},  with  the  large  fifth  nerve  arising  from  it, 
and  the  middle  peduncles  of  the  cerebellum  (M.  Ped)  running  into  the  cere- 
bellum (A).  The  rounder  bodies  anterior  to  the  pons  are  the  corpora  quad- 
rigemina  (C'q),  at  the  sides  of  which  are  the  crura  cerebri  and  the  origins  of 
the  third  and  fourth  nerves.  The  optic  and  olfactory  nerves  are  in  front. 
The  under  surfaces  of  the  cerebrum  (.Cb)  and  cerebellum  (A)  constitute 
the  remainder  of  the  drawing.      (From  a  i)reparation  by  I'.   M.  Spurney. ) 


258 


HUMAN   PHYSIOLOGY, 


parts  of  the  brain.    The  optic  thalamus,  as  its  name  signifies,  is 
intimately  associated  with  the  optic  nerves. 

Another  important  collection  of  nerve  cells  occurs  in  the 
corpora  quadrigemina.  These  exist  as  four  rounded  swellings, 
two  on  either  side,  just  where  the  superior  peduncles  of  the  cere- 
bellum come  together.  Their  nerve  cells  serve  as  distributing 
centers  for  visual  and  auditory  impulses,  carried  to  them  through 
tracts  of  nerve  fibers  connected  with  the  optic  and  auditory 


Fig.  46. — Vertical  transverse  section  of  human  brain.  Below  is  a  section 
of  the  pons  (P)  showing  the  fibers  which  connect  the  brain  stem  and  cere- 
brum radiating  up  through  the  internal  capsule  (/C),  which  is  bounded 
mesially  by  the  optic  thalmus  (T),  and  laterally  by  the  corpus  striatum  (i). 
The  third  (III-V)  and  lateral  ventricles  (.LV)  of  the  brain  are  seen  in  the 
center  (black).  The  thicltness  of  the  grey  matter  and  the  infolding  of  the 
surfaces,  as  convolutions,  should  be  noted.  (From  a  preparation  by  P.  M. 
Spurney. ) 

nerves.     The  corpora  quadrigemina  are  usually  more  developed 
in  the  brain  of  the  lower  animals  than  in  that  of  man. 

The  branial  Nerves. — On  account  of  the  introduction  of 
the  new  structures  described  above  there  is  no  regularity  in  the 


THE   CRANIAL  NERVES. 


259 


arrangement  of  the  grey  matter  in  the  brain  stem  as  there  is  in 
the  cord.  Instead  of  forming  horns,  the  grey  matter  is  scat- 
tered in  colonies  or  nuclei,  many  of  which  are  centers  for 
the  fibers  of  the  cranial  nerves.  Some  of  these  fibers  are,  of 
course,  afferent  and  some  efferent.  Since  many  of  the  cranial 
nerves  are  connected  with  the  nose,  mouth  and  teeth,  it  is  im- 
portant for  us  to  learn  something  concerning  the  location  of 
their  centers  and  the  general  function  of  the  nerves.  There  are 
twelve  pairs  of  cranial  nerves,  and  the  last  ten  of  these  originate 
from  the  grey  matter  of  the  medulla,  pons  or  midbrain.  The 
following  list  indicates  the  general  functions  of  the  nerves: 


1.  Olfactory. 

2.  Optic. 

3.  Oculo  motor. 

4.  Trochlear. 

6.  Abducens. 

5.  Trigeminal. 

7.  Facial. 

8.  Auditory. 


9.  Glosso-pharyn- 
geal. 

10.  Vagus. 


11.  Spinal  accessory. 


12.  Hypoglossal. 


nerve  of  smell, 
nerve  of  sight. 

nerves    to   the    mus- 
cles of  the  eyeball. 

sensory  nerve  of 

face, 
main  motor  nerve  of 

face  muscles, 
nerve  of  hearing  and 

of    semicircular 

canals, 
motor  nerve  of  phar- 
ynx, sensory  nerve 

of  taste, 
efferent  and  afferent 

nerve    to    various 

viscera, 
mainly    blends    with 

vagus 


motor   nerve   for 
tongue  muscles 


It  is  important  to  note  that,  like  the  spin 
the  cranial  nerves  are  composed  of  two  roots. 


arises  from  fore- 
brain. 

arises  from  fore- 
brain. 

arise  from  midbrain. 

arises  mainly  in 

pons, 
arises    in    pons    and 

medulla, 
arises  in  pons. 


arises    mainly    in 
medulla. 

arises  in   medulla. 


arises  with  vagus 
except  spinal  por- 
tion, which  extends 
down  into  spinal 
cord. 

arises  in  medulla. 

al  nerves,  many  of 
motor  and  sensory, 


260,  HUMAN  PHYSIOLOGY. 

each  having  its  own  center.  This  fact  justifies  the  statement 
which  we  have  already  made  that  the  brain  stem  is  really  an  up- 
ward prolongation  of  the  spinal  cord,  and  just  as  we  saw  that 
each  posterior  root  of  the  spinal  cord  is  characterized  by  pos- 
sessing a  ganglion,  so  also  is  there  a  ganglion  in  the  sensory 
divisions  of  the  cranial  nerves.  This  ganglion,  however,  is  often 
difficult  to  find.  The  nerve  cells  which  compose  it  unite  with 
the  fibers  of  the  sensory  root  by  a  T-shaped  junction,  and  the 
fibers  terminate  by  synapsis  around  the  cells  of  the  sensory 
nuclei.  The  ganglion  of  the  fifth  nerve  is  the  Gasserian.  Those 
for  the  eighth  are  the  ganglia  found  in  the  cochlea  and  internal 
auditory  meatus  (Scarpa's  ganglion).  The  ganglia  of  the  ninth 
and  tenth  nerves  are  situated  along  the  course  of  the  nerves. 

The  approximate  position  of  the  various  ganglia  will  be  best 
learned  by  consultatipn  of  the  accompanying  diagram  (Fig.  47). 

■In  -the  brain  stem  there  are  three  sensory  or  afferent  nuclei,  a 
long,  combined  one  for  the  ninth,  tenth  and  eleventh  nerves,  ex- 
tending practically  from  the  upper  to  the  lower  limits  of  the 
medulla,  one  for  the  eighth  in  the  center  of  the  pons,  and  a 
very  long  one  for  the  fifth,  extending  from  near  the  upper  limit 
of  the  pons  down  into  the  spinal  cord.  The  motor  or  efferent 
nuclei  for  the  third,  fourth,  sixth  and  twelfth  nerves  are  com- 
posed of  cells  shaped  like  those  of  the  anterior  horn  of  the  spinal 
cord.  They  lie  near  the  middle  line  and  extend  throughout 
the  whole  length  of  medulla  and  pons.  The  motor  nuclei  of  the 
fifth,  seventh,  ninth,  tenth  and  eleventh  lie  outside  the  above. 

It  is  important  that  the  following  functions  of  these  nerves  be 
studied  by  dental  students : 

The  Third  Nerve. — The  third  nerve  controls:  (1)  the  mus- 
cles of  accommodation  inside  the  eye;  (2)  all  of  those  which 
are  attached  to  the  outside  of  the  eyeball,  except  the  muscle 
which  moves  it  out  (external  rectus),  and  the  one  which  rotates 
it  down  and  out  (the  superior  oblique)  ;  and  (3)  the  elevator 
muscle  of  the  eyelids  (levator  palpebrse).  When  the  third  nerve 
is  paralyzed,  the  symptoms  are  therefore:  (1)  drooping  of  the 
eyelid  (ptosis)  so  that  the  chin  is  tilted  upward  when  the  pa- 
tient looks  at  anything;  (2)  inability  to  see  clearly  unless  when 


fig.  47. — Diagram  of  the  dorsa!  aspect  of  the  medulla  and  pons  showing 
the  floor  of  the  fourth  ventric'.e  with  the  nuclei  of  origin  of  the  cranial 
nerves.  (After  Sherrington.)  The  sensory  nuclei  are  colored  red  and  are 
numbered  on  the  left  of  the  diagram,  the  motor,  blue  and  numbered  on  the 
right.  The  peduncles  of  the  cerebellum — 8.  (superior),  M.  (middle),  and  /.. 
(inferior),  are  shown  cut  across.  CO.,  corpora  quidrigimina.  The  above 
nuclei  are  of  course  present  on  both  sides. 


THE   CRANIAL  NERVES.  261 

objects  are  at  a  distance  (long  sight)  ;  (3)  squint  of  the  eye  so 
that  it  is  directed  outward  and  domiward. 

Such  a  paralysis  of  the  eye  is  sometimes  accompanied  by  a 
partial  hemiplegia  (see  p.  271)  of  the  opposite  side  of  the  body, 
thus  idicating  that  some  destructive  lesion  (haemorrhage,  de- 
structive tumour)  exists  on  one  side  of  the  midbrain,  so  that  it 
involves  the  nucleus  of  origin  of  the  third  nerve  and  also  the 
P3^ramidal  fibers  lying  near.  Since  the  fibers  of  the  third  nerve 
do  not  cross  to  the  opposite  side,  but  those  of  the  pyramids  do 
(see  p.  243),  we  get  a  crossed  or  alternating  paralysis.  Some- 
times only  one  part  of  the  third  nerve  may  be  paralyzed,  for 
example,  that  portion  going  to  the  muscles  of  accommodation. 

The  Fourth  and  Sixth  Nerves. — The  fourth  and  sixth  nerves 
supply  the  two  extra-ocular  muscles  not  supplied  by  the  third, 
viz.,  the  superior  oblique  (fourth)  and  the  external  rectus 
(sixth),  respectively. 

The  Fifth  Nerve.— -The  fifth  nerve  is  the  largest  of  the 
cranial  nerves,  and  is  a  representative  mixed  nerve.  It  supplies 
the  teeth.  The  motor  hranch  runs  to  the  muscles  of  mastica- 
tion, the  tensor  nmsele  of  the  palate,  the  mylohyoid  muscle  (in 
the  floor  of  the  mouth)  and  the  anterior  belly  of  the  digastric. 
These  last  two  mentioned  muscles  pull  the  hyoid  bone  and  there- 
fore the  root  of  the  tongue  upward  and  forward  during  the  act 
of  swallowing.  Both  mastication  and  swallowing  are  seriously 
impaired  when  this  nerve  is  paralyzed.  The  sensory  fibers  are 
connected  with  the  receptors  for  all  the  conmion  sensations  of 
the  head  and  face.  As  already  explained,  they  are  connected 
with  the  nerve  cells  of  the  Gasserian  ganglion,  which  is  lodged 
in  a  depression  near  the  apex  of  the  petrous  portion  of  the 
temporal  bone.  Shortly  after  leaving  this  ganglion,  the  nerve 
divides  into  three  branches:  (1)  Upper  or  ophthalmic,  carry- 
ing the  sensory  nerve  fibers  for  the  conjunctiva,  the  mucous 
membrane  of  the  nasal  fossae,  and  the  skin  of  the  eyebrow,  fore- 
head and  nose.  (2)  Middle  or  superior  maxillary,  supplying 
the  meninges,  the  lower  eyelid,  the  skin  of  the  side  of  the  nose 
and  upper  lip  and  all  the  teeth  and  gums  of  the  upper  jaw.  (3) 
Inferior  maxillary,  supplying  the  teeth  and  gums  of  the  lowei* 


262  HUMAN  PHYSIOLOGY. 

jaw,  the  skin  of  the  temple  and  external  ear,  the  lower  part  of 
the  face  and  the  lower  lip. 

Eelationship  OB'  THE  FiFTH  Nerve  TO  THE  Teeth. — In  any  in- 
flammatory condition  of  the  teeth,  the  terminations  of  the  sen- 
sory fibers  become  stimulated,  causing '  extreme  pain.  This  is 
toothache.  The  relationship  of  the  fifth  nerve  to  the  teeth  ex- 
plains why  disturbance  in  the  latter  should  often  cause  the  pain 
to  be  referred  not  to  the  tooth  that  is  involved,  but  to  some  skin 
area  on  the  face.  This  is  called  referred  pain.  The  skin  areas 
corresponding  to  the  different  teeth  have  been  worked  out  by 
Head,  and  are  indicated  in  the  accompanying  diagrams  (Figs. 
48  and  49).  Not  only  may  the  pain  be  referred  to  the  skin  area, 
but  this  itself  may  become  hypersensitive.  There  is,  moreover, 
in  each  area  usually  a  maximal  spot  at  which  the  pain  and  ten- 
derness are  most  marked. 

The  sensory  nerve  endings  in  the  teeth  are  all  of  the  nature  of 
pain  receptors;  there  are  no  temperature  or  tactile  receptors, 
these  latter  sensations  being  particularly  developed  in  the  tongue 
and  lips  (see  p.  244).  The  pain  receptors  of  the  teeth,  like  those 
of  the  cornea,  react  practically  in  full  intensity  to  every  strength 
of  stimulus.  This  explains  why  a  small  degree  of  irritation,  as 
that  due  to  caries,  may  cause  as  painful  a  toothache  as  an  in- 
tense irritation.  As  we  have  already  explained,  the  purpose  of 
painful  or  nocuous  sensation  is  protective,  causing,  for  example, 
withdrawal  of  the  irritated  portion  of  the  body  or  some  move- 
ment of  offense  (see  p.  251).  In  the  case  of  the  teeth  it  serves  as 
a  warning  that  something  must  be  done  to  arrest  whatever 
condition  is  causing  it.  The  enamel  and  cement  are  devoid  of 
nerve  endings,  which,  however,  are  very  abundant  in  the  pulp, 
and  probably  also  in  the  dental  tubules  (Mummery).  An  inert, 
sensationless  exterior  covering,  a  highly  sensitive  center,  and 
between  these  a  moderately  sensitive  tissue,  describes  the  sensi- 
tiveness of  a  tooth.  The  sensitiveness  of  the  pulp  is  so  great  as 
to  suggest  that  it  is  partly  of  the  nature  of  a  highly  specialized 
noci-receptor,  just  as  the  taste  buds  and  olfactory  epithelium- 
are  specialized  receptors  for  taste  and  smell.  The  sensitiveness 
of  the  teeth  diminishes  with  advancing  age. 


Fionlo-nasal      area 
lary   incisors). 


(maxil- 


Naso-labial    area    (maxillary 
canine  and  first  premolar). 


The  points  of  maximum   intensity 


Maxillary      area      (maxillary 

second    premolar    and    first 

molar). 
Mental      area       (mandibular 

incisors,    canine    and    first 

jiremolar). 
are   ringed. 


Kijr.  48. — Diagram  to  show  areas  of  referred  pain  in  distriljution  of  fifth 
nerve  due  to  affections  of  the  various  teeth  (Front  view).  (  Krom  drawing 
by  T.  Wingate  Todd.) 


THE   CRANIAL   NERVES.  263 

The  fifth  nerve  is  very  commonly  the  seat  of  neuralgia,  which 
may  affect  one  or  all  of  its  branches.  This  is  called  "tic 
douloureux"  or  tri-facial  neuralgia.  The  attacks  come  in 
spasms,  and  besides  the  excruciating  pain,  there  is  often  twitch- 
ing of  the  muscles  or  flushing  of  the  skin  of  the  face.  Pressure 
at  the  points  where  the  branches  of  the  nerve  come  out  of  the 
skull,  as  at  the  supra  or  infra-orbital  notches,  is  usually  espe- 
cially painful  in  tic.  An  unhealthy  condition  of  the  teeth  is 
often  responsible  for  the  symptoms,  but  if  dental  treatment 
and  general  medical  care  do  not  remove  the  neuralgia,  it  is 
usually  advisable  to  ffut  out  a  portion  of  the  nerve  or  even  to 
remove  the  entire  Gasserian  ganglion. 

Sometimes  the  fifth  nerve  becomes  paralyzed,  causing  anes- 
thesia involving  the  area  of  its  distribution.  Tingling,  numb- 
ness or  neuralgic  pains  often  precede  the  anesthesia.  Since  the 
conjunctiva  loses  its  sensitiveness,  particles  of  dust,  etc.,  are  not 
removed  from  the  eye  by  the  tears  so  that  tliey  set  up  inflam- 
mation, which  may  develop  and  cause  ulceration  of  the  cornea. 
For  the  same  reason,  or  perhaps  because  the  nerve  independently 
controls  the  nutrition  of  tissues,  the  gums  and  cheeks  may  be- 
come ulcerated  and  the  teeth  loosened.  Partial  loss  of  taste  and 
inability  to  smell  pungent  vapors,  which  act  on  sensory  nerves, 
are  also  common  symptoms. 

The  Seventh  Nerve. — The  seventh  nerve  is  purely  motor  in 
function.  All  the  facial  muscles,  except  those  concerned  in 
mastication,  the  platysma  of  the  neck,  the  posterior  belly  of  the 
digastric  and  one  of  the  muscles  of  the  middle  ear  (the  sta- 
pedius) are  supplied  by  it.  On  account  of  its  tortuous  course 
the  seventh  nerve  is  peculiarly  liable  to  inflammation  and  com- 
pression. Thus  tumors  or  inflammation  located  at  the  base  of 
the  brain  may  involve  that  portion  running  between  the  upper 
end  of  the  medulla  "oblongata  and  the  internal  auditory  meatus, 
where  the  nerve  enters  the  aqueduct  of  Fallopius.  In  this 
region  it -is  likely  to  become  involved  when  there  is  disease  cf 
the  internal  ear  or  mastoid  sinus  (mastoiditis).  After  its  exit 
from  the  skull  (by  the  stylomastoid  foramen)  its  close  association 
with  the  parotid  gland  renders  it  liable  to  be  involved  in  eel- 


264  HUMAN   PHYSIOLOGY. 

lulitis  of  this  gland,  and  on  account  of  its  superficial  position, 
it  may  be  injured  by  blows  on  the  side  of  the  head.  Quite  com- 
monly the  seventh  nerve  becomes  the  seat  of  inflammation  after 
exposure  to  a  draught,  as  by  sitting  at  an  open  window.  The 
paralysis  is  almost  always  one-sided.  The  eyelid  on  the  affected 
side  cannot  be  properly  closed,  a  chink  remains  and  the  eyeball 
becomes  rotated  upward,  thus  showing  the  sclera.  On  smiling 
or  showing  the  teeth  the  mouth  is  drawn  up  on  the  healthy  side, 
causing  a  triangular  opening  because  the  lips  do  not  become 
separated  on  the  paralyzed  side.  Articulation  is  difficult  and 
such  acts  as  whistling  and  blowing  are  impossible.  Because  of 
paralysis  of  the  buccinator  muscle,  food  collects  between  the 
cheek  and  gums.  The  distortion  of  the  face  is  much  more  pro- 
nounced in  old,  than  in  young  persons;  indeed  in  the  ease  of 
the  latter  the  paralysis  may  be  overlooked  until  speaking  or 
laughing  is  attempted. 

The  Eighth  or  Auditory  Nerve. — The  eighth  or  auditory 
nerve  is  composed  of  two  branches,  the  one  called  cochlear,  con- 
nected with  the  organ  of  Corti  (see  p.  291),  which  collects  sound 
waves,  and  the  other,  called  vestibular,  with  the  semicircular 
canals  which,  by  the  movements  of  the  fluid  contained  in  them, 
record  changes  in  the  position  of  the  head  (see  p.  276).  Both 
branches,  being  sensory,  are  connected  with  ganglia  situated  in 
or  near  the  internal  ear  (ganglion  spirale  for  the  cochlear  di- 
vision and  ganglion  of  Scarpa  for  the  vestibular).  Paralysis 
of  the  auditory  nerve  causes  a  degree  of  deafness  wliich  is  more 
profound  than  that  due  to  disease  of  the  middle  ear,  for  in  the 
latter  case  a  tuning  fork  can  be  heard  when  the  end  of  it  is 
applied  to  the  skull  or  is  held  in  the  teeth,  which  is  not  the  case 
when  the  nerve  is  diseased.  When  the  eighth  nerve  becomes 
irritated  (as  by  inflammation  of  the  ear,  or  a  general  condition 
such  as  migraine,  epilepsy,  etc.),  various  kinds  of  sounds  are 
heard.  This  is  called  tinnitus.  It  is  not  infrequently  followed 
by  deafness. 

The  Ninth  or  Glosso-pharyngeal  Nerve. — This  nerve  is 
partly  motor  and  partly  sensory.  The  motor  fibers  supply  the 
muscles  of  the  pharynx  and  most  of  those  of  the  soft  palate. 


'J'emporal  area  ( max  ilia  ry 
second  premolar). 

MandiVjular  area  (maxillary 
second  and  third  premo- 
lars). 


Hyoid  area   (mandibular  sec- 
ond    premolar ;     first     and  - 
second    molars). 

S  u  p  e  r  i  o  r    laryngeal    area 
(mandibular  third  molar). 


The  points  of  maximum  intensity  are  ringed. 

Fig.  49. — Diagram  to  show  areas  of  referred  pain  in  distribution  of  fifth 
nerve  due  to  affections  of  the  various  teeth  (Side  view).  (From  drawing 
by  T.  Wingate  Todd.) 


THE   CRANIAL  NERVES.  265 

Tlie  sensory  fibers  carry  impulses  of  common  sensation  and  of 
taste  from  the  root  of  the  tongue,  the  neighboring  portions  of 
the  pharynx,  the  tonsils,  the  soft  palate,  and  the  pillars  of  the 
fauces.  This  nerve  does  not  commonly  become  the  seat  of  local 
lesions. 

The  Tenth  or  Vagus  Nerve. — This  is  the  main  cerebrospinal 
nerve  supplying  the  viscera  and  it  is  both  motor  and  sensory 
in  function.  We  shall  see  later  that  the  nerves  to  the  viscera 
belong  to  the  so-called  autonomic  system,  which  is  distinguished 
from  the  somatic  by  two  main  facts,  one  anatomical  and  one 
functional.  Tlie  anatomical  difference  is  that  every  nerve  fiber 
becomes  connected  through  synapses  with  nerve  cells  located 
peripherally  (i.  e.,  near  the  end  of  the  nerve),  and  the  axons  of 
the  cells  continue  tlie  impulse  on  to  the  structure ;  the  functional 
difference  is  that  the  autonomic  fibers,  as  their  name  indicates, 
control  automatically-acting  or  involuntary  functions  instead  of 
voluntary  movements,  as  is  the  case  with  the  ordinary  or  somatic 
cerebrospinal  nerve  fibers. 

The  most  important  of  the  vagus  autonomic  fibers  run  to  the 
heart  (see  p.  185),  the  a?sophagus  (p.  57),  the  stomach  (p.  60) 
and  the  intestines  (p.  79).  The  vagus  also  contains  afferent 
fibers  which  have  their  cell  stations  in  ganglia  situated  in  the 
trunk  of  the  nerve.  These  fibers  carry  sensory  impulses  par- 
ticularly from  the  larynx  and  lungs  (p.  219).  Further  details 
regarding  the  functions  controlled  by  the  vagus  are  fully  given 
in  the  references  indicated  above.  When  the  vagus  nerve,  or 
its  center,  is  the  seat  of  paralysis,  swallowing  is  seriously  in- 
terfered with,  and  food  is  liable  to  pass  into  the  larynx  and 
cause  pneumonia.  Various  forms  of  paralysis  of  the  vocal  cords 
may  also  result  from  paralysis  of  the  vagus. 

The  Eleventh  or  Spinal  Accessory  Nerve. — The  eleventh 
or  spinal  accessory  is  entirely  an  efferent  nerve,  one  part  of 
it,  the  accessory,  being  derived  from  the  same  column  of  nerve 
cells  as  the  vagus  and  being  really  a  part  of  this  nerve;  the 
other  arises  from  the  cells  of  the  anterior  horn  of  the  spinal 
cord  in  the  upper  cervical  region  atifl  supplies  the  trapezius  and 
sterno-mastoid  muscles. 


266  HUMAN   PHYSIOLOGY. 

The  Twelfth  or  Hypoglossal  Nerve. — The  twelfth  or  hypo- 
glossal nerve  is  entirely  efferent,  being  the  motor  nerve  of  the 
tongue  muscles  and  of  most  of  the  muscles  attached  to  the  hyoid 
bone.  When  it  is  paralyzed,  as  in  bulbar  paralysis,  swallowing 
of  food  becomes  impossible,  the  tongue  cannot  be  protruded 
and  soon  atrophies  because  of  the  removal  of  the  trophic  in- 
fluence of  the  nerve  cells.  Rarely  the  paralysis  is  unilateral, 
but  this  is  because  of  lesions  higher  up  in  the  nervous  system 
than  the  medulla  and  so  situated  that  they  destroy  the  con- 
nection of  the  fibers  which  run  from  the  higher  motor  centers 
in  the  cerebrum  to  the  hypoglossal  nucleus.  Such  lesions  neces- 
sarily involve  fibers  of  -the  same  type  running  to  the  nerve  cells 
of  the  spinal  cord,  so  that  hemiplegia  (p.  248)  accompanies  and 
is  on  the  same  side  as  the  tongue  paralysis.  "When  a  patient 
with  such  a  lesion  attempts  to  put  out  the  tongue,  it  is  directed 
towards  the  affected  side  but  it  shows  no  atrophy. 


CHAPTER  XXVIII. 

THE  NERVOUS  SYSTEM   (Cont'd). 

The  Brain. 

The  first  question  which  naturally  arises  is,  what  influence 
does  the  brain  have  on  the  reflex  movements  produced  through 
the  spinal  cord?  These  influences  may  be  summarized  as  fol- 
lows: 

1.  The  brain  enables  the  animal  to  will  that  a  particular 
movement  shall  or  shall  not  take  place,  irrespective  of  the  stimu- 
lation of  spinal  reflexes.  Much  of  this  influence  of  the  brain  is 
of  course  voluntary  in  nature,  but  some  of  it  is  subconscious  or 
involuntary.  In  general  it  may  be  ,said  that  the  cerebrum, 
through  the  pyramidal  tracts,  usually  exercises  a  damping  or 
inhibitory  influence  on  the  spinal  reflexes.  It  is  for  this  reason 
that  the  reflex  response  to  a  certain  stimulus  is  usually  much 
more  pronounced  in  a  spinal,  as  compared  with  a  normal  animal. 
For  example,  it  is  impossible  to  bring  about  the  scratch  reflex 
in  many  normal  dogs,  whereas  it  is  always  present  in  spinal 
animals. 

In  man  this  restraining  influence  of  the  pyramidal  tracts  on 
spinal  reflexes  is  very  evident  in  the  case  of  knee-jerk,  which, 
it  will  be  remembered,  is  the  extension  of  the  leg  which  occurs 
when  the  stretched  patellar  tendon  is  tapped.  Ordinarily  the 
kick  is  moderate  in  degree,  but  in  patients  whose  pyramidal 
tracts  are  diseased,  as  in  spastic  paraplegia,  it  becomes  very 
pronounced. 

2.  The  brain,  being  the  receiving  station  for  the  projicient 
sensations  (p.  279),  sight,  hearing  and  smell,  adds  greatly  to 
the  number  of  afferent  pathways  by  which  reflex  actions  can 
be  excited. 

3.  Since  in  higher  animals  all  the  afferent  impulses  usually 

267 


268  HUMAN   PHYSIOLOGY. 

travel  through  the  brain  (p.  248),  many  nerve  centers  become 
more  or  less  involved  in  the  reflex  actions,  so  that  a  much  higher 
degree  of  co-ordination  than  that  seen  in  a  spinal  animal  attends 
the  muscular  response.  For  example,  some  of  these  afferent 
impulses  reach  the  cerebellum,  whose  function,  as  we  shall  see, 
is  to  strengthen  some  impulses  and  weaken  others,  so  that  a 
more  perfect  movement  results. 

4.  The  animal  becomes  conscious  not  only  of  the  nature  and 
place  of  application  of  the  sensory  stimulus  itself,  but  of  the 
degree  to  which  it  has  moved  its  muscles  in  response. 

The  Functions  of  the  Cerebrum. 

The  complicated  movements,  such  as  those  involved  in  the 
scratch  reflex,  which  we  have  seen  that  a  spinal  animal  can  carry 
out  in  the  paralyzed  region  after  shock  has  passed  away,  become 
more  and  more  numerous  and  complicated  as  the  higher  centers 
are  left  in  connection  Math  the  spinal  cord.  That  is  to  say,  the 
higher  up  in  the  cerebrospinal  axis  the  section  is  made,  the 
more  capable  does  the  part  of  the  animal  below  the  section  be- 
come to  peform  complicated  niovements.  The  important  centers 
in  the  medulla,  pons  and  mesencephalon  add  their  influence  to 
those  of  the  spinal  cord  itself,  so  that  integration  becomes  more 
comprehensive.  If  the  cut  is  made  above  the  level  of  the  pons, 
in  other  words,  if  the  cerebral  hemispheres  alone  be  discon- 
nected from  the  rest  of  the  cerebrospinal  axis — decerebration, 
as  it  is  called — we  obtain  an  animal  possessing  all  the  reflex 
actions  that  are  necessary  for  its  bare  existence,  although  it  is 
of  course  incapable  of  feeling  or,  if  the  basal  ganglion  be  also 
destroyed,  of  seeing  or  hearing.  It  becomes  a  mere  automaton : 
it  breathes,  the  blood  circulation  is  normal,  it  can  walk  or  run  . 
or  swim,  it  swallows  food  if  the  reflex  act  of  swallowing  be 
stimulated  by  placing  the  food  in  the  mouth,  but  it  has  not  the 
sense  to  take  food  itself  even  when  this  is  placed  near  it.  All 
the  mental  processes  are  absent;  it  has  no  memory,  no  volition, 
no  likes  and  dislikes.  By  seeing  that  it  takes  food,  it  has  been 
possible  to  keep  such  a  decerebrated  dog  alive  for  eighteen 
months,  and  the  lower  we  descend  in  the  animal  scale,  the  easier 


THE  FUNCTIONS  OF  THE  CEREBRUM.  269 

it  becomes  to  perform  the  operation  and  to  keep  the  animal  alive. 
In  higher  animals,  such  as  monkeys,  however,  life  is  impossible 
without  the  cerebrum,  thus  supporting  the  conclusion,  which  we 
have  already  drawn  (see  p.  243),  that  the  cerebrum  comes  to 
be  a  necessary  part  of  every  reflex  action  in  the  higher  animals. 

Cerebral  Localization. — The  various  functions  of  the  cere- 
brum are  located  in  different  portions  of  it.  This  localization 
of  cerebral  functions  has  been  very  extensively  studied  during 
recent  years,  partly  by  experimental  work  on  the  higher  mam- 
malia and  partly  by  clinical  studies  bn  man.  Careful  observa- 
tions are  made  of  the  behavior  of  the  various  functions  of  tlie 
animal  either  after  removal  or  destruction  of  a  portion  of  the 
cerebrum,  or  during  its  stimulation  by  the  electric  current.  Im- 
portant additions  to  our  knowledge  of  cerebral  localization  are 
also  being  made  by  correlating  the  symptoms  observed  in  insane 
persons  with  the  lesions  wliicli  are  revealed  by  post-mortem 
examination. 

It  has  been  found  that  there  are  roughly  three  areas  on  the 
cerebrum  with  distinct  and  separate  functions   (Fig.  50). 

I.  In  the  portions  of  the  cerebrum  which  lie  in  front  of  tlie 
ascending  frontal  convolutions — prefrontal  region — are  located 
the  centers  of  the  intellect  (thought,  ideation,  memory,  etc.). 
This  part  of  the  cerebrum  is  accordingly  by  far  the  best  de- 
veloped in  man;  it  is  much  less  so  in  the  apes  and  monkeys, 
becomes  insignificant  in  the  dog,  and  still  more  so  in  the  rabbit. 
It  has  been  destroyed  by  accident  in  man  with  the  result  that 
all  the  higher  mental  powers  vanished. 

II.  The  next  portion  includes  roughly  the  region  of  the 
cerebrum  bordering  upo2i  the  Rolandic  fissure  (i.  e.,  the  ascend- 
ing frontal  and  ascending  parietal  convolutions).  Here  are 
located  the  highest  centers  for  the  movements  of  the  various 
parts  of  the  body.  Microscopic  examination  of  the  grey  matter 
reveals  tlie  presence  of  large  triangular  nerve  cells,  which  com- 
municate by  synapses  (sec  p.  241)  with  the  afferent  fibers  that 
carry  the  sensory  impulses,  whose  course  from  the  posterior 
spinal  roots  we  have  already  traced  (p.  246).  From  each  of 
these  cells  an  efferent  fiber  runs  to  join  the  pyramidal  tract 


270 


HUMAN   PHYSIOLOGY, 


(p.  248),  and  thus  connect  with  the  anterior  horn  cells  of  the 
spinal  cord. 

In  the  Rolandic  area,  as  it  is  called,  is  therefore  situated  the 
cerebral  link  in  the  chain  of  neurones  (see  p.  249)  through 
which  the  ordinary  movements  of  the  body  take  place.  Such 
movements  may  be  set.  agoing,  either  by  stimulation  of  the 
Rolandic  nerve  cells  through  afferent  fibers — a  pure  reflex — or 
by  impulses  coming  to  them  from  the  centers  of  volition  situated 


Fig.  50. — Cortical  centers  in  man.  Of  the  three  sliaded  areas  bordering  on 
the  Rolandic  fissure  (,Rol.),  the  most  anterior  is  the  precentral  associational 
area,  the  middle  one  is  the  motor  area  (the  position  of  the  body  areas  are 
indicated  on  it),  and  the  most  posterior  is  the  sensory  area,  to  the  cells  of 
which  the  fillet  fibers  proceed.  The  centers  for  seeing  and  hearing  are  also 
shown.  The  unshaded  portion  in  front  of  the  Rolandic  area  is  the  precentral ; 
the  portions  behind,  the  parietal  and  temperosphenoidal. 


in  the  prefrontal  convolutions.  Or,  again,  the  nerve  cell,  at  the 
same  time  that  it  receives  a  sensory  impulse  coming  up  from 
the  spinal  cord,  may  receive  one  from  the  prefrontal  convolu- 
tions which  may  either  interdict  or  greatly  modify  the  reflex 
response.  Every  possible  muscular  group  in  the  body  has  a 
center  of  its  own  in  the  Rolandic  area,  the  determination  of  the 
exact  location  of  these  centers  being  one  of  the  achievements 
of  modern  medical  science.     Thus,  if  we  stimulate  with  a  finely 


'  THE  FUNCTIONS  OF  THE  CEREBRUM.  271 

graded  electric  stimulus,  say,  the  center  of  the  thumb,  it  -will 
be  found  that  the  thumb  undergoes  a  sIqw,  purposeful,  co-ordi- 
nated movement ;  and  so  on  for  every  other  center.  Or,  if  in- 
stead of  stimulating,  we  cut  away  one  of  the  centers  and  allow 
the  animal  to  recover  from  the  immediate  effects  of  the  opera- 
tion, it  will  be  found  that  all  the  more  finely  co-ordinated  move- 
ments of  the  corresponding  part  of  the  body  have  disappeared, 
although  gross  reflex  movements  may  be  possible,  because  the 
spinal  reflexes  are  still  intact.  If  the  entire  Rolandic  area  on 
one  side  is  removed,  the  muscles  of  the  opposite  side  of  the  bod}^, 
except  those  of  the  trunk,  become  completely  paralyzed  for 
some  time,  after  which,  however,  particularly  in  the  case  of 
young  animals,  the  paralysis  becomes  recovered  from,  thus  in- 
dicating that  some  other  portions  of  the  brain  have  assumed 
the  function  of  the  destroyed  centers.  If  the  stimulus-  is  a  very 
strong  one,  the  movements  do  not  remain  conflued  to  the  cor- 
responding muscle  group,  but  they  spread  on  to  neighboring 
groups  until  ultimately  the  whole  extremity  or  perhaps  even 
all  the  muscles  of  that  side  of  the  body  are  involved. 

These  experimental  results  find  their  exact  counterpart  in 
clinical  experience.  Thus  when  some  center  becomes  irritated 
by  pressure  on  it  of  some  tumor  growing  in  the  membranes  of 
the  brain  (meningeal  tumor),  or  by  a  piece  of  bone,  as  in  de- 
pressed fracture  of  the'  skull,  or  by  blood  clot,  convulsive  at- 
tacks (known  as  Jacksonian  epilepsy)  are  common.  The 
first  sign  of  such  an  attack  is  usually  some  peculiar  sensation 
(aura)  affecting  the  part  of  the  body  which  corresponds  to  the 
irritated  area;  the  muscles  of  this  part  begin  to  twitch  and 
more  muscles  are  involved,  until  ultimately  all  those  of  the  cor- 
responding half  of  the  body  become  contracted.  There  is,  how- 
ever, no  loss  of  consciousness,  as  there  is  in  true'  epilepsy. 
The  evident  cause  of  these  symptoms  has  clearly  indicated  the 
proper  treatment  for  such  cases,  namely,  surgical  removal  of 
the  cause  of  irritation.  For  this  purpose  a  very  careful  study 
is  first  of  all  made  of  the  exact  group  of  muscles  in  which  the 
convulsions  originate;  the  location  of  the  area  on  the  cerebrum 
is  thus  ascertained  and  a  trephine  hole  is  made  in  the  correspond- 


272  HITMAN   PHYSIOLOGY. 

ing  part  of  the  cranium  and  through  this  hole  the  tumor  or 
blood  clot  is  removed. 

III.  These  so-called  motor  areas  are  of  course  also  sensory 
areas  in  the  sense  that  the  afferent  stimuli  which  come  up  from 
the  spinal  eord  run  to  them.  They  are  really  sensori-motor 
centers.  For  some  of  the  more  highly  specialized  proficient 
sensations,  such  as  vision  and  hearing  (see  p.  279),  there  are, 
however,  special  centers.  These,  along  with  an  extensive  field 
of  associational  or  junctional  grey  matter,  constitute  the  third 
main  division  of  the  cerebral  cortex  and  occupy  the  greater 
part  of  the  parietal,  the  temporosphenoidal  and  the  occipital 
lobes.  The  visual  is  the  most  definite  of  these  centers.  Thus 
if  the  occipital  lobe  be  removed  or  destroyed  by  disease  on  one 
side,  the  corresponding  half  of  each  retina  becomes  blind.  It 
is  by  studying  the  exact  nature  of  the  involvement  of  vision  in 
such  eases  that  the  physician  is  able  to  locate  the  position  of  a 
tumor,  etc. 

The  center  for  Jiearing  is  in  the  temporosphenoidal  lobe,  but 
its  location  is  not  very  definite. 

It  will  be  seen,  however,  that  the  visual  and  auditory  centers 
take  up  but  a  small  part  of  this  third  division  of  the  cerebrum, 
the  most  of  it  being  occupied  by  associational  areas.  The  nerve 
cells  of  these  areas  do  not,  like  those  of  the  motor  and  sensory 
centers,  send  fibers  which  run  as  pyramidal  or  optic  fibers  to 
some  lower  nerve  center,  but  only  to  other  cerebral  centers, 
which  they  serve  to  link  together.  They  are  specialized  to  serve 
as  junction  points  for  all  the  receiving  and  discharging  centers 
of  the  cerebrum,  so  that  all  actions  may  be  properly  correlated  or 
integrated.  These  junctional  centers  thus  perform  the  great 
function  of  adapting  every  action  of  the  entire  animal  to  some 
definite  purjDose.  Together  with  the  nerve  cells  in  the  prefrontal 
areas,  the  associational  cells  represent  the  highest  development 
of  cerebral  integration,  so  that  we  find  the  areas  in  which  they 
lie  becoming  more  and  more  pronounced,  the  higher  we  ascend 
the  animal  scale. 

The  Mental  Process. — The  impression  received  by  the  visual 
center  when  a  young  animal  looks  for  the  first  time  at,  say  a 


THE  FUNCTIONS  OF  THE  CERF^RUM.  273 

bell,  becomes  stored  away  in  nerve  cells  lying  in  or  close  to  that 
center,  and  when  the  bell  is  moved  sound  memories  are  likewise 
stored  in  the  auditory  center.  At  first  these  remain  as  isolated 
memory  impressions  and  the  animal  is  unable  to  associate  the 
sight  with  the  sound  of  the  bell-  But  later,  with  repetition,  the 
visual  and  the  auditory  centers  become  linked  together,  through 
nerve  cells  and  fibers  which  occupy  the  associational  areas,  so 
that  the  invocation  of  one  memory  is  followed  by  association 
with  others.  It  is  evident  that  the  intricacy  of  this  interlace- 
ment of  different  centers  will,  in  large  part,  determine  the  in- 
tellectual development  of  the  animal,  and  the  possibility  of  his 
learning  to  judge  of  all  the  consequences  that  must  follow  every 
impression  which  he  receives  or  every  act  which  he  performs. 
In  man  these  associational  areas  are  very  poorly  developed  at  the 
time  of  birth,  so  that  the  human  infant  can  perform  but  a  few 
acts  for  itself.  Everything  has  to  be  learned,  and  the  learning 
process  goes  hand  in  hand  with  development  of  the  associational 
areas,  which  proceeds  through  many  years.  On  the  other  hand, 
most  of  the  lower  animals  are  born  with  the  associational  areas 
already  laid  down  and  capable  of  very  little  further  increase, 
so  that,  although  much  more  able  than  the  human  infant  to 
fend  for  itself  at  birth,  the  lower  animal  does  not  afterwards 
develop  mentally  to  the  same  extent. 

The  practical  application  of  these  facts  concerning  the  func- 
tions of  different  areas  of  the  cerebrum  is  in  the  study  of  mental 
diseases.  To  serve  as  an  example  we  may  take  aphasia.  This 
means  inability  to  interpret  sights  or  sounds  or  to  express  the 
thoughts  in  language.  In  the  former  variety — called  sensory 
aphasia — ^the  patient  can  see  or  hear  perfectly  well,  but  fails 
to  recognize  that  he  has  seen  or  heard  the  object  before.  He 
fails  to  recognize  a  printed  word  (word  blindness)  or  to  in- 
terpret it  when  spoken  (word  deafness).  The  lesion  responsible 
for  this  condition  is  located  in  the  associational  areas  and  not 
in  the  centers  themselves.  In  the  other  variety,  called  motor 
aphasia,  the  patient  understands  the  meaning  of  sounds  or 
sights,  of  spoken  or  written  words,  but  is  unable  to  express  his 
thoughts  or  impressions  in  language.    The  lesion  in  this  case  in- 


274  HUMAN   PHYSIOLOGY. 

volves  some  of  the  centers  concerned  in  the  higher  control  of 
the  muscles  which  are  used  in  speech,  and  very  commonly  it  is 
situated  in  the  left  side  of  the  cerebrum.  In  all  three  forms  of 
aphasia  there  is  more  or  less  decrease  in  the  mental  powers. 

Cerebellum. 

The  afferent  impulses  set  up  by  stimulation  of  the  nerves  of 
the  skin  in  a  spinal  animal,  and  due  therefore  to  changes  in 
the  environment,  after  entering  the  spinal  cord  travel  to  the 
various  centers  in  the  cord.  Although  complicated  movements 
may  result  (e.g.,  the  scratch  reflex),  there  is  an  entire  absence 
of  the  power  of  maintaining  bodily  equilibrium,  and  the  animal 
cannot  stand  because  the  muscles  are  not  kept  in  the  degree  of 
tone  which  is  necessary  to  keep  the  joints  properly  stiffened. 
A  similar  inability  to  maintain  the  center  of  gravity  of  the 
body  results  from  removal  of  the  cerebellum,  or  small  brain, 
which  it  will  be  remembered  is  situated  dorsal  to  the  medulla 
and  pons,  with  which  it  is  connected  by  three  peduncles.  The 
cerebellum  consists  of  two  lateral  hemispheres  and  a  median 
lobe  called  the  vermis.  The  remarkable  infolding  of  the  grey 
matter  which  composes  its  surface,  and  the  large  number  of 
nuclei  which  lie  embedded  in  its  central  white  matter  are  struc- 
tural peculiarities  of  the  cerebellum. 

The  immediate  results  of  removal  of  the  cerebellum  consist  in 
extreme  restlessness  and  inco-ordination  of  movements.  The 
animal  is  constantly  throwing  itself  about  in  so  violent  a  man- 
ner that  unless  controlled  it  may  dash  itself  to  death.  Gradually 
the  excitement  gets  less,  until  after  several  weeks  all  that  is 
noticed  is  that  there  is  a'  condition  of  muscular  weakness  and 
tremor,  and  difficulty  in  maintaining  the  body  equilibrium. 
Quite  similar  symptoms  occur  when  the  cerebellum  is  diseased 
in  man  (as  by  the  growth  of  a  tumor)  j  the  condition  being 
called  cerebellar  ataxia,  and  being  characterized  by  the  uncer- 
tain gait  which  is  like  that  of  a  drunken  man. 

These  observations  indicate  that  the  function  of  the  cerebellum 
is  to  harmonize  the  actions  of  the  various  muscular  groups,  so 


THE  FUNCTIONS  OP  THE   CEREBELLUM.  275 

that  any  disturbance  in  the  center  of  gravity  of  the  body  may  be 
subconsciously  rectified  by  appropriate  action  of  the  various 
muscular  groups.  It  evidently  represents  the  nerve  center  hav- 
ing supreme  control  over  other  nerve  centers,  so  that  these  may 
not  bring  about  such  movements  as  would  disturb  the  equiii- 
hrimn  of  the  animal. 

In  order  that  the  cerebellum  may  perform  this  function  it 
must,  however,  be  informed  of  two  things.  In  the  first  place,  it 
must  know  the  existing  state  of  contraction  of  the  muscles  and 
the  tightness  of  the  various  tendons  that  pull  upon  the  joints, 
and  in  the  second,  it  must  know  the  exact  position  of  the  center 
of  gravity  of  the  body. 

Information  of  the  condition  of  the  muscles  and  tendons  is 
supplied  through  the  nerves  of  muscle  sense,  which  run  in 
every  muscle  nerve  and  are  connected  in  the  muscles  with 
peculiar  -sensory  nerve  terminations  called  muscle  spindles. 
When  the  muscles  contract,  or  the  tendons  are  put  on  the  stretch, 
these  spindles  are  compressed  and  sensory  or  afferent  stimuli 
pass  up  the  nerves  of  muscle  sense,  enter  the  cord  by  the  pos- 
terior roots  and  reach  the  cerebellum  by  way  of  the  lateral  col- 
umns (see  p.  249). 

Information  regarding  the  center  of  gravity  of  the  body  is 
supplied  through  the  vestibular  division  of  the  eighth  nerve, 
which,  it  will  be  recalled,  is  connected  with  the  semicircular  can- 
als and  vestibule.  In  these  structures  are  membranous  tubes  or 
sacs  containing  a  sensory  organ  (called  the  crista  or  macula 
acoustica),  which  consists  essentially  of  groups  of  columnar  cells 
furnished  with  very  fine  hair-like  processes  at  their  free  ends 
and  connected  at  the  other  end  with  the  fibers  of  the  eighth 
nerve.  The  hair-like  processes  float  in  the  fluid  which  is  con- 
tained in  the  membranous  canals  or  sacs.  This  fluid  does  not, 
however,  completely  fill  these  structures,  so  that  it  moves  when- 
ever the  head  is  moved.  This  movement  affects  the  hair-like 
processes  and  thus  sets  up  nerve  impulses  which  are  carried 
to  the  cerebellum. 

To  make  the  hair  cells  of  this  receiving  apparatus  capable 
of  responding  to  every  possible  movement  of  the  bead,  it  is, 


276 


HUMAN  PHYSIOLOGY. 


however,  evident  that  there  must  be  some  definite  arrangement 
of  the  tubes.  This  is  provided  for  in  the  disposition  of  the  semi- 
circular canals  in  three  planes,  namely,  a  horizontal  and  two 
vertical  (Fig.  51).  Taken  together  the  three  canals  form  a  struc- 
ture which  looks  somewhat  like  a  chair,  the  horizontal  canals 
being  the  seat  of  the  chair  and  the  two  vertical  canals  joining 
together  to  form  its  back  and  arms.  The  back  of  each  chair  is 
directed  inwards  so  that  they  are  back  to  back.  At  one  end  of 
each  canal  is  a  swelling,  the  ampulla,  in  which  the  sensory  nerve 


Fig.    51. — The  semicircular  canals  of  the   ear,   showing  their  arrangement 
In  the  three  planes  of  space.      (From  Howell's  Physiology.) 


apparatus  above  described  is  located.  It  is  evident  that  when 
the  head  is  moved  in  any  direction  the  fluid  in  some  of  these 
canals  will  be  set  in  motion.  It  is  this  movement  of  the  fluid 
which  stimulates  the  hair  cells.  That  this  is  really  the  function 
of  the  semicircular  canals  is  proved  by  the  fact  that  if  they  are 
irritated  or  destroyed,  grave  disturbances  occur  in  the  bodily 
movements.  This  is  what  occurs  in  Meniere's  disease,  in  which 
attacks  of  giddiness,  often  severe  enough  to  cause  the  patient 
to  fall,  and  accompanied  by  extreme  nausea,  are  the  chief  symp- 
toms,  the  lesion  being  a   chronic   inflammation   involving  the 


THE  SYMPATHETIC  NERVOUS  SYSTEM.  277 

semicircular  canals.  It  is  believed  by  some  that  the  constant 
movements  of  the  fluid  in  the  semicircular  canals  is  the  cause 
of  sea  sickness.  The  unusual  nature  of  these  movements  causes 
confusion  in  the  impressions  transmitted  to  the  cerebellum  from 
the  canals,  but  after  a  while  the  cerebellum  may  become  accus- 
tomed to  them  and  the  sea  sickness  passes  away. 

The  Sympathetic  Nervous  System. 

Along  with  the  vagus  and  one  or  two  less  prominent  cere- 
brospinal nerves,  the  sympathetic  constitutes  the  autonomic 
nervous  system,  so-called  because  it  has  to  do  with  the  innerva- 
tion of  automatically  acting  structures,  such  as  the  viscera,  the 
glands  and  the  blood  vessels.  The  characteristic  structural  fea- 
ture of  the  nerves  of  this  system  is  that  they  are  connected 
with  nerve  ganglia  located  outside  the  central  nervous  system. 
In  these  ganglia  the  nerve  fibers  run  to  nerve  cells,  around  which 
they  form  synapses,  thus  permitting  the  nerve  impulse  to  pass 
on  to  the  cell,  which  then  transmits  it  to  its  destination  along  its 
own  axon  (see  p.  241).  Before  arriving  at  the  ganglion  in 
which  the  synapsis  is  formed,  the  fibers  are  called  pregan- 
glionic; after  they  leave,  they  are  called  postganglionic.  A 
preganglionic  fiber  may  run  through  several  ganglia  before  it 
becomes  changed  to  a  postganglionic  fiber.  In  the  case  of  the 
vagus  and  other  cerebral  autonomic  nerves,  the  ganglia  are 
often  situated,  as  in  the  heart  (see  p.  185),  at  the  end  of  the 
nerve,  but  in  the  case  of  the  sympathetic  itself,  they  are  more 
numerous,  and  are  mainly  situated  at  the  sides  of  the  vertebral 
column,  where,  together  with  the  connecting  fibers,  they  form  a 
chain — the  sympathetic  chain — which  can  easily  be  seen  on 
opening  the  thorax  and  displacing  the  heart  and  lungs. 

Two  fine  branches  connect  each  of  the  spinal  nerves  with 
the  corresponding  sympathetic  ganglion.  It  is  through  one  of 
these  branches  that  the  sympathetic  chain  receives  its  fibers 
from  the  spinal  cord.  Through  the  other,  fibers  run  from  the 
ganglion  to  the  spinal  nerve.  Some  of  the  sympathetic  ganglia 
are  situated  at  a  distance  from  the  spinal  cord ;  the  ganglia 
which  compose  the  solar  and  hypogastric  plexuses  are  examples. 


278  -  HUMAN  PHYSIOLOGY. 

In  the  thorax,  the  uppermost  ganglion  is  very  large  and  is 
called  the  stellate  ganglion.  Its  postganglionic  fibers  constitute 
the  vasomotor  nerves  of  the  blood  vessels  of  the  anterior  ex- 
tremity, and  the  sympathetic  fibers  to  the  heart.  Some  pregan- 
glionic fibers  run  through  the  stellate  ganglion  to  pass  up  the 
neck  as  the  cervical  sympathetic,  their  cell  station  being  in  the 
superior  cervical  ganglion.  They  act  on  the  pupil  (dilating  it), 
on  the  salivary  glands  (causing  vasoconstriction  and  stimulating 
glandular  changes),  and  on  the  blood  vessels  of  the  head,  face 
and  mucosa  of  the  inside  of  the  mouth. 

From  about  the  fifth  dorsal  vertebra  downwards,  branches  run 
from  the  sympathetic  chain  on  each  side  to  become  collected 
into  a  large  nerve  called  the  great  splanchnic,  which  passes  down 
by  the  pillars  of  the  diaphragm  into  the  abdomen  and  runs  to 
the  ganglia  of  the  coeliac  plexus.  This  nerve  supplies  all  of 
the  blood  vessels  of  the  intestines  and  other  abdominal  viscera. 
Its  action  on  these  vessels  has  already  been  described  (see  p. 
191).  It  also  carries  nerve  impulses  for  the  control  of  the  move- 
ments of  the  stomach  and  intestines  and  for  some  .of  the  digestive 
glands.  In  the  abdomen  the  sympathetic  chain  gives  off  branches, 
which  form  the  pelvic  nerves  and  supply  the  blood  vessels  of 
the  lower  extremity.  It  is  important  to  note  that  the  connections 
between  the  sympathetic  system  and  the  cerebrospinal  axis  are 
limited  to  the  spinal  nerve  roots  between  the  second  thoracic  and 
the  second  lumbar.  The  results  which  follow  stimulation  of 
the  sympathetic  system  are  exactly  like  those  which  are  pro- 
duced by  injections  of  adrenalin  (see  p.  130). 


CHAPTER  XXIX. 

THE  SPECIAL  SENSES, 

The  sensory  nerve  terminations,  or  afferent  receptors,  that 
are  scattered  over  the  skin  are  affected  by  stimuli  which  come  in 
actual  contact  with  the  surface  of  the  body.  In  order  that  the 
stimuli  transmitted  from  a  distance,  such  as  those  of  light,  sound 
and  smell,  or  the  projicient  sensations  as  they  are  called,  may 
be  appreciated  by  the  nervous  system,  specifically  designed  or- 
gans, called  the  organs  of  special  sense,  are  required.  These 
organs  collect  the  stimuli  in  such  a  way  as  to  cause  them  to  act 
effectively  on  receptors  which  have  been  especially  adapted  to 
react  to  them. 

Although  not  really  a  projicient  sensation,  taste  is  conven- 
iently considered  along  with  the  above. 

Vision. 

Light  is  due  to  vibration  of  the  ethereal  particles  that  oc- 
cupy space.  The  vibrations  occur  at  right  angles  to  the  rays 
of  light,  and  these  travel  at  high  velocity  in  straight  lines  from 
the  source  of  the  light.  The  rate  of  vibration  of  the  rays  is  not 
always  the  same,  and  on  this  difference  depends  the  color  of  the 
light,  red  light  vibrating  much  slower,  and  its  waves  being 
accordingly  much  longer,  than  those  of  violet  light.  The  termi- 
nations of  the  optic  nerve,  the  retina,  have  been  specially 
developed  to  receive  the  light  waves.  But  in  order  that  a 
comprehensive  picture  of  everything  that  is  to  be  seen  may  be 
projected  on  the  retina,  an  optical  apparatus,  consisting  of  the 
cornea  and  lens,  is  situated  in  front  of  it.  The  retina  and  the 
optical  apparatus  are  built  into  a  globe — the  eyeball — which, 
pivoting  on  the  attachment  of  the  optic  nerve,  can  be  so  moved 
that  images  from  different  parts  of  the  field  of  vision  may  be 

279 


280  HUMAN  PHYSIOLOGY. 

focused  in  turn  on  the  retina.     These  movements  are  effected 
by  the  so-called  ocular  muscles. 

There  are,  therefore,  three  functions  involved  in  the  act  of 
seeing:  (1)  That  of  the  retina,  in  reacting  to  light.  (2)  That  of 
the  cornea,  etc.,  in  focusing  the  light.  (3)  That  of  the  ocular 
muscles,  in  moving  the  eyeball. 

The  Optical  Apparatus  of  the  Eye. 

It  will  readily  be  seen  that  the  eye ,  is  constructed  on  much 
the  same  principle  as  a  photographic  camera,  the  retina  being 
like  the  sensitive  plate.  There  is,  however,  an  important  dif- 
ference in  the  manner  by  which  objects  at  varying  distances  are 
brought  to  a  focus  on  the  sensitive  surface  in  these  two  cases : 
in  the  camera,  it  is  done  by  adjusting  the  distance  between  the 
lens  and  the  focusing  screen;  in  the  eye,  it  is  done  by  varying 
the  convexity  of  the  lens. 

In  order  to  understand  how  the  optical  apparatus  works,  it 
is  necessary  to  know  something  about  the  refraction  of  light. 
"When  a  ray  of  light  passes  from  one  medium  to  another,  it  be- 
comes bent  or  refracted.  When  it  passes  from  air  to  water  or 
glass,  for  example,  it  becomes  refracted  so  that  the  angle  which 
the  refracted  ray  makes  with  the  perpendicular  to  the  surface 
is  less  than  that  of  the  entering  ray.  In  other  words,  the  ray 
becomes  bent  towards  the  perpendicular.  The  greater  the  dif- 
ference in  density  between  the  two  media,  the  greater  is  the' 
difference  between  the  two  angles.  A  figure  expressing  the  ratio 
between  these  two  angles  is  called  the  index  of  refraction.  If 
the  ray  of  light  leaves  the  denser  medium  by  a  surface  which 
is  parallel  with  that  by  which  it  entered  (as  in  passing  through 
a  pane  of  glass),  it  will  be  refracted  back  to  its  old  direction, 
but  if,  as  in  a  prism,  it  leaves  the  denser  medium  by  a  surface 
which  forms  an  angle  with  that  by  which  it  entered,  the  original 
refraction  will  be  exaggerated.  If  two  prisms  be  placed  with 
their  broad  ends  together,  parallel  rays  of  light  coming  from  a 
certain  direction  will  be  bent  so  that,  on  leaving  the  prisms, 
they  meet  somewhere  behind  them.  Two  prisms  so  arranged  are 
virtually  the  same  as  a  biconvex  lens.     It  is  plain  that  the 


VISION. 


281 


focusing  power  of  such  a  lens  will  depend  on  two  things :  first, 
its  index  of  refraction,  and,  secondly,  the  curvature  of  its  sur- 
faces. 

A  considerable  part  of  the  actual  refraction  of  the  rays 
which  enter  the  eye  is  accomplished  at  the  curved  surface  of 
the  cornea,  a  smaller  degree  of  refraction  taking  place  at  the 
lens  itself.  The  reason  for  this  is  that  the  refractive  index 
from  air  to  cornea  is  much  greater  than  that  between  the  lens 
and  the  humors  of  the  eye  in  which  the  lens  is  suspended,  these 
humors  and  the  cornea  having  very  much  the  same  refractive 
indices.  The  entering  rays  are,  therefore,  refracted  at  two 
places  in  the  eye,  namely,  at  the  anterior  surface  of  the  cornea 
and  on  passing  through  the  lens. 


Fig.  52. — Formation  of  image  on  retina.   O.A.  is  the  optic  axis. 


Accommodation  of  the  Eye  for  Near  Vision. — When  the  eye 
is  at  rest,  its  optical  system  is  of  such  a  strength  that  parallel 
rays,  i.  e.,  rays  that  are  reflected  from  objects  at  a  distance,  are 
brought  to  a  focus  exactly  on  the  retina.  The  picture  thus 
formed  is,  however,  upside  down  for  the  same  reason  that 
it  is  so  on  the  screen  of  a  camera  (Fig.  52).  When  the  object 
looked  at  is  so  near  that  the  rays  reflected  from  it  are  divergent 
when  they  enter  the  eye,  it  becomes  necessary,  if  the  image  is 
still  to  be  focused  on  the  retina,  that  some  adjustment  take  place 
in  the  optical  system  of  the  eye.  This  could  happen  in  one 
of  two  ways,  either  by  lengthening  the  distance  between  the 
lens  and  the  retina  (the  method  used  in  a  camera),  or  by  in- 
creasing the  convexity  of  the  lens.    The  former  process  cannot 


282 


HUMAN   PHYSIOLOGY. 


occur  in  the  eye,  but  the  second  is  rendered  possible  by  bulging 
of  the  anterior  surface  of  tJie  lens.  There  are  several  ways  by 
which  this  bulging  of  the  lens  can  be  proven  to  occur.  Thus, 
if  the  eye  of  a  person  who  is  looking  at  some  distant  object  be 
inspected  from  the  side  of  the  head,  that  is  to  say,  in  profile, 
it  is  easy  to  note  the  exact  position  of  the  iris,  which,  with  the 
'pupil  in  its  center,  hangs  as  a  circular  curtain  just  in  front  of 
the  lens  (Fig.  53).     If  the  person  is  now  told  to  regard  some 


Fig.  53.- — Section  through  the  anterior  portion  of  the  eye:  C,  the  cornea; 
I,  the  iris  (note  the  circular  muscular  fibers  cut  across  at  the  margin)  ;  L, 
the  lens  ;  Cij  the  ciliary  process  ;  S,  the  suspensory  ligament ;  8cl,  the  scler- 
otic or  outer  protective  coat  of  the  eye.  (From  a  preparation  by  P.  M.  Spur- 
ney. ) 


object  held  close  to  him,  it  will  be  seen  that  the  iris  is  pushed 
forward  nearer  to  the  cornea.  That  this  is  really  due  to  a  bulg- 
ing of  the  anterior  surface  of  the  lens  can  be  shown  by  placing 
a  candle  to  one  side  and  a  little  in  front  of  the  head  and  then, 
from  the  other  side,  viewing  the  images  of  the  candle  flame 
which  are  cast  on  the  eye.  It  will  be  seen  that  one  image  occurs 
at  the  anterior  surface  of  the  cornea,  and  another,  less  distinct, 
at  the  anterior  surface  of  the  lens.     This  image  from  the  lens 


VISION.  283 

will  be  seen  to  move  forward — that  is  to  say,  closer  to  the  image 
at  the  cornea — when  the  person  shifts  his  gaze  from  a  distant 
to  a  near  object.  By  using  optical  apparatus  for  measuring  the 
size  of  the  images,  the  degree  to  which  the  convexity  of  the  lens 
has  increased,  as  a  result  of  the  bulging,  can  be  accurately 
measured. 

This  change  in  the  convexity  of  the  lens  depends  on  the  fact 
that  it  is  composed  of  a  ball  of  transparent  elastic  material, 
which  is  kept  more  or  less  flattened  antero-posteriorly  because  of 
its  being  slung  in  a  capsule  w^hich  compresses  it.  The  edges  of 
the  capsule  are  attached  to  a  fine  ligament  (the  suspensory  liga- 
ment), which  runs  backwards  and  outwards  to  become  inserted 
into  the  ciliar}^  processes  (Fig.  53).  These  processes  exist  as 
thickenings  of  the  anterior  portion  of  the  choroid,  or  pigment 
coat  of  the  eye,  and  they  can  be  moved  forwards  by  the  action 
of  a  small  fan-shaped  muscle,  called  the  ciliary  muscle,  which 
at  its  narrow^  end  originates  in  the  corneo-seleral  junction,  and 
runs  back  to  be  attached,  by  its  wide  end,  to  the  ciliary  pro- 
cesses. When  this  muscle  is  at  rest,  the  ciliary  processes  lie  at 
such  a  distance  from  the  edges  of  the  lens  that  the  suspensory 
ligament  is  put  on  the  stretch.  When  the  ciliary  muscle  con- 
tracts, it  pulls  the  ciliary  processes  forward,  thus  slackening 
the  suspensory  ligament  and  removing  the  tension  on  the  capsule 
of  the  lens,  with  the  result  that  the  latter  bulges  because  of  its 
elasticity.  The  ability  of  the  lens  to  become  accommodated  for 
near  vision  depends,  therefore,  first,  on  the  elasticity  of  the 
lens,  and  secondly,  on  the  action  of  the  ciliary  muscle.  Inter- 
ference with  either  of  these  renders  accommodation  faulty.  For 
example,  the  lens,  along  with  the  other  elastic  tissues  of  the 
body  [e.  g.,  the  arteries  (p.  175)],  becomes  less  elastic  in  old 
age,  thus  accounting  for  the  "long-sightedness"  (or  presbyopia) 
which  ordinai'ily  develops  at  this  time.  Paralysis  of  the  ciliary 
muscle  produces  the  same  effect  in  even  more  marked  degree, 
which  explains  the  utter  inability  to  bring  about  any  accommo- 
dation after  treating  the  eye  with  atropin,  which  is  given  for 
this  purpose  before  testing  the  vision  in  order  to  find  out  the 
strength  of  lenses  required  to  correct  for  errors  in  refraction. 


284  HUMAN  PHYSIOLOGY. 

The  Function  of  the  Pupil. — ^Every  optical  instrument  con- 
tains a  so-called  diaphragm,  which  is  a  black  curtain  having  a 
central  aperture  whose  diameter  can  be  altered  to  any  required 
size.  The  object  of  this  is  to  prevent  all  unnecessary  rays  of 
light  from  entering  the  optical  instrument,  thus  materially  in- 
creasing the  distinctness  of  the  image.  In  the  eye,  this  function 
is  performed  by  the  iris  with  the  pupil  in  its  center.  The  size 
of  the  pupil  is  altered  by  the  action  of  two  sets  of  muscle  fibers 
in  the  iris.  One  of  these  runs  in  a  circular  manner  around  the 
inner  edge  of  the  iris;  by  contracting  it  causes  constriction  of 
the  pupil,  an  event  which  occurs,  along  with  the  bulging  of  the 
lens,  during  accommodation  for  near  vision.  The  other  layer  of 
fibers  runs  in  a  radial  manner,  and  by  contracting  causes  dila- 
tation of  the  pupil.  This  occurs  in  partial  darkness,  or  when  the 
eye  is  at  rest  (although  not  during  sleep).  The  circular  fibers 
are  supplied  by  the  third  nerve,  and  the  radial  fibers  by  the 
sympathetic.  Under  ordinary  conditions  both  muscles  are  in  a 
state  of  tonic  contraction  (see  p.  253),  so  that  the  actual  size 
of  the  pupil  at  any  moment  is  the  balance  between  two  opposing 
muscular  forces.  This  renders  its  adjustment  in  size  very  sensi- 
tive. For  example,  it  can  become  dilated  either  by  stimulation 
of  the  sympathetic  (which  occurs  when  any  irritative  tumor 
affects  the  cervical  sympathetic  nerve),  or  by  paralysis  of  the 
third  nerve  (as  by  giving  atropin).  Conversely,  constriction  of 
the  pupil  may  be  the  result  of  stimulation  of  the  third  nerve  (as 
by  a  tumor  at  the  base  of  the  brain)  or  paralysis  of  the  sympa- 
thetic. 

These  local  conditions  acting  on  the  afferent  nerves  to  either 
pupil  are  not  nearly  so  often  called  into  play  as  conditions  acting 
reflexly  on  both  eyes  at  the  same  time. 

Certain  of  the  afferent  impulses  which  call  these  reflexes  into 
play  travel  by  the  optic  rrerve  to  the  nerve  centers  for  the  pupil, 
such  for  example  as  the  stimulus  set  up  by  light  falling  on  the 
retina.  The  afferent  pathway  concerned  in  the  contraction  of 
the  pupil,  which  occurs  in  accommodation,  must,  on  the  other 
hand,  be  a  different  one  because  in  the  disease  locomotor  ataxia 
(see  p,  254),  the  pupil  contracts  on  accommodation,  but  does  not 


VISION.  285 

do  so  when  light  is  thrown  into  the  eyes.  The  nerve  centers  for 
the  pupil  are  very  sensitive  to  general  nervous  conditions,  thus 
accounting  for  the  dilatation  of  the  pupil  which  occurs  during 
fright  or  other  emotions,  or  pain.  The  pupils  are  contracted  in 
the  early  stages  of  asphyxia  or  anesthesia,  as  in  the  early  stages 
of  nitrous  oxide  administration,  but  they  become  dilated  when 
the  anesthesia  or  asphyxia  becomes  profound.  Their  condition 
helps  to  serve  as  a  gauge  of  the  depth  of  anesthesia. 

Imperfections  of  Vision. — The  optical  system  of  the  eye  is 
not  perfect.  Some  of  these  imperfections  exist  in  every  eye, 
whilst  others  are  only  occasional.  The  errors  in  every  eye  are 
those  known  as  spherical  and  chromatic  aberration.     Spherical 


Fig.  54. — A,  spherical  aberration.  The  rays  which  strike  the  margins  of 
the  lens  are  brought  to  a  focus  before  those  striking  near  the  center.  B, 
Chromatic  aberration.  The  ray  of  white  light  (W)  is  dissociated  by  the 
lens  into  the  spectral  colors,  of  which  those  at  the  red  end  {R)  are  not 
brought  to  a  focus  so  soon  as  those  at  the  violet  end  (V). 

aberration  (Fig.  54),  occurs  because  the  edges  of  the  lens  have 
a  higher  refractive  power  than  the  center,  so  that  the  image  on 
the  retina  is  surrounded  by  a  halo  of  overfocused  rays.  Chro- 
matic aberration  is  due  to  the  fact  that  white  light,  on  passing 
through  the  lens,  suffers  some  decomposition  into  its  constituent 
colored  rays  (the  rainbow  colors),  of  which  certain  ones  (viz., 
those  towards  the  violet  end  of  the  spectrum)  come  to  a  focus 
sooner  than  others  (viz.,  those  towards  the  red  end),  thus  creat- 
ing a  colored  edge  on  the  focused  image.  These  errors  are 
greatly  minimized,  although  not  entirely  removed,  by  the  pupil, 
which  cuts  out  the  peripheral  rays. 

The  occasional  errors  are  long-sightedness  or  hypermetropia. 


286 


HUMAN   PHYSIOLOGY. 


short-sightedness  or  myopia,  and  astigmatism  (Fig.  55).  Hyper- 
metropia  is  due  to  the  eyeball  being  too  short  so  that  the  focus 
of  the  image  is  behind  the  retina.  The  error  is  corrected  by 
prescribing  convex  glasses.     Myopia  is  due  to  the  opposite  con- 


Fig.  55. — Errors  in  refraction:  E  shows  the  formation  of  the  image  on 
the  retina  in  the  normal  or  emmetropic  eye  ;  H  shows  the  condition  in  long- 
sight,  or  hypermetropia,  where  the  eyeball  is  too  short ;  M  shows  the  condi- 
tion in  short-sight,   or  myopia,   where  the   eyeball   is  too   long. 


ditiou,  that  is,  the  eyeball  is  too  long,  so  that  the  focus  occurs 
in  front  of  it.  Concave  glasses  correct  it.  Astigmatism  is  due  to 
the  lens  or  cornea  being  of  unequal  curvature  in  its  different 


VISION.  287 

meridians.  This  causes  the  rays  of  light  in  one  plane  to  be 
brought  to  a  focus  before  those  in  other  planes,  so  that  the  two 
hands  of  a  clock,  when  they  are  at  right  angles  to  each  other, 
cannot  be  seen  distinctly  at  the  same  iiistant,  although  they  can 
be  successively  focused.  A  certain  amount  of  astigmatism  exists 
in  every  eye,  but  when  it  becomes  extreme,  it  is  necessary  to 
correct  it  by  prescribing  glasses  which  are  astigmatic  in  the 
opposite  meridian  to  that  of  the  eye.  Such  glasses  are  called 
cjdindrical. 

Astigmatism  may  occur  along  with  either  myopia  or  hyper- 
metropia,  and  when  any  of  these  errors  is  only  slight  in  degree, 
the  patient  may  be  able,  by  efforts  of  accommodation,  to  over- 
come the  defect.  The  strain  thus  thrown  on  the  ciliary  muscle 
is,  however,  quite  commonly  the  cause  of  severe  headache.  The 
correction  of  the  errors  should  never  be  left  to  untrained  per- 
sons, but  a  proper  oculist  should  be  consulted,  since  it  is  usually 
necessary  to  give  atropin  so  that  the  accommodation  may  be 
paralyzed  and  the  exact  extent  of  the  error  measured.  The  use 
of  improper  glasses  may  aggravate  the  defect  of  vision  and  do 
much  more  harm  than  good. 

The  Sensory  Apparatus  of  the  Eye. 

The  Functions  of  the  Retina. — The  image  which  is  formed  on 
the  retina  by  the  optical  system  of  the  eye  sets  up  nerve  im- 
pulses which  travel  by  the  optic  nerve  to  the  visual  center  in 
the  occipital  lobes  of  the  cerebrum  (see  p.  272),  where  they  are 
interpreted.  Microscopic  examination  of  the  retina  has  shown 
that  it  consists  of  several  layers  of  structures,  the  innermost 
being  of  fine  nerve  fibers  which  arise  from  an  adjacent  layer  of 
large  nerve  cells,  and  the  outermost  of  peculiar  rod  or  cone- 
shaped  cells,  called  the  rods  and  cones.  Between  the  layer  of 
large  cells  and  the  layer  of  rods  and  cones  are  several  layers 
composed  of  other  nerve  cells  and  of  interlacements  of  the  pro- 
cesses of  cells  and  nerve  fibers.  The  rods  and  cones  are  the 
.structures  acted  on  by  light,  the  other  layers  of  the  retina  being 
for  the  purpose  of  connecting  the  rods  and  cones  with  the  large 
nerve  cells  from  which  the  fibers  of  the  innermost  layer  arise. 


288  HUMAN"  PHYSIOLOGY. 

The  fibers  all  converge  to  the  optic  disc,  which  is  a  little  to  the 
inside  of  the  posterior  pole  of  the  eyeball.  At  this  point  the 
fibers  of  the  nerve  fiber  layer  bend  backwards  at  right  angles 
and  run  into  the  optic  nerve,  thus  crowding  out  the  other  layers 
and  causing  the  existence  of  a  Hind  spot,  which  can  be  readily 
demonstrated  by  closing  one  eye,  say  the  left,  and  with  the  other 
regarding  the  letter  B  in  the  next  line.     Although  the   S  is 


also  distinctly  visible  in  most  positions,  yet  if  the  book  be 
moved  towards  and  away  from  the  eye,  the  S  will  become  in- 
visible at  a  certain  distance  corresponding  to  that  at  which  the 
rays  from  it  are  impinging  upon  the  blind  spot.  As  we  alter 
the  distance  of  the  book  from  the  eye,  the  line  of  vision,  or 
visual  axis,  being  fixed  on  the  B,  the  image  of  the  S  travels 
from  side  to  side  across  the  inner  or  nasal  half  of  the  retina, 
and  at  a  certain  position  strikes  the  optic  disc.  Ordinarily  we 
are  unaware  of  the  blind  spot,  partly  because  we  have  two  eyes 
and,  the  blind  spot  being  towards  the  nasal  side  of  each  retina, 
the  image  of  an  object  does  not  fall  on  it  in  both  eyes  at  the 
same  time ;  and  partly  because  we  have  learned  to  disregard  it. 
The  area  or  extent  of  the  blind  spot  may  become  so  increased,  as 
by  excessive  smoking,  that  it  becomes  noticeable. 

At  another  portion  of  the  retina  called  the  fovea  centralis,  all 
the  layers  become  thinned  out  except  that  of  the  rods  and  cones, 
especially  the  cones.  This,  as  we  should  expect,  is  by  far  the 
most  sensitive  portion  of  the  retina,  and  is  indeed  the  portion  on 
which  we  cause  the  image  to  be  focused  when  we  desire  to  see 
an  object  clearly.  The  remainder  of  the  retina  is  only  suffi- 
ciently sensitive  to  give  us  a  general  impression  of  what  we  are 
looking  at.  Thus  when  we  view  a  landscape,  we  can  see  only  a 
small  portion  clearly  at  one  time,  although  we  have  a  general 
impression  of  the  whole.  The  portion  which  we  see  clearly  is 
that  which  is  focused  on  the  fovea,  and  we  keep  moving  our 
eyes  in  all  directions  so  that  every  part  of  the  landscape  may  in 
turn  be  properly  seen.  We  see  with  the  fovea  what  the  rest  of 
the  retina  informs  us  there  is  to  be  seen. 


VISION.  289 

The  Movements  of  the  Eyeballs. — In  order  that  we  may  be 
enabled  to  move  our  eyes  so  as  to  see  objects  in  different  posi- 
tions in  the  visual  field,  the  eyeballs  are  provided  with  six  little 
muscles,  four  recti  and  two  obliques.  These  muscles  are  in- 
nervated by  the  third,  fourth  and  sixth  nerves  (see  p.  259).  The 
images  in  the  two  eyes  cannot  of  course  fall  on  anatomically 
identical  parts  of  the  retinae,  but  they  fall  on  parts  that  are 
physiologically  identical.  Thus,  an  object,  say  on  the  right  of 
the  field  of  vision,  will  cause  an  image  to  fall  on  the  nasal  side 
of  the  right  retina  and  on  the  temporal  side  of  the  left  retina. 
We  do  not,  however,  see  two  objects  because  by  experience  we 
have  come  to  learn  that  these  are  corresponding  points  on  the 
retinae.  When  an  object  is  brought  near  to  the  eye,  the  two 
eyeballs  must  converge  so  as  to  bring  the  visual  axes  on  to  the 
corresponding  points.  This  convergence  of  the  eyeballs  con- 
stitutes the  third  change  occurring  in  the  eyes  during  accom- 
modation for  near  vision,  the  other  two  being,  as  we  have  seen, 
bulging  of  the  lens  and  contraction  of  the  pupil.  It  is  interest- 
ing that  these  three  changes  are  controlled  by  the  third  nerve. 
If  anything  happens  to  throw  one  of  the  images  on  to  some 
other  portion  of  one  retina,  double  vision  is  the  result.  This 
condition  of  diplopia,  as  it  is  called,  can  be  brought  about,  vol- 
untarily, by  pressing  on  one  eyeball  at  the  edge  of  the  eye,  or 
it  may  occur  as  a  result  of  paralysis  or  incoordinate  action  of 
one  or  more  of  the  ocular  muscles.  This  occurs  in  certain  in- 
toxications, as,  for  example,  that  produced  by  alcohol. 

Just  as  in  the  case  of  errors  of  refraction,  e.  g.,  astigmatism, 
slight  degrees  of  diplopia  may  cause  symptoms  that  are  more 
distressing  than  when  marked  diplopia  exists,  because  we  try 
to  correct  for  slight  errors  and  the  effort  causes  pain  (headache) 
and  fatigue,  whereas  with  extreme  errors  we  do  not  try  to  correct 
but,  in.stead,  we  learn  to  disregard  entirely  the  image  in  one 
eye.  Whenever  the  incoordination  of  ocular  movement  is  per- 
manent, as  when  due  to  shortening  of  one  of  the  muscles,  it  is 
called  strabismus.  This  condition  is  usually  congenital,  and  can 
often  be  rectified  by  a  surgical  operation. 

Judgments  of  Vision. — Besides  these  purely  physiological 


290  HUMAN  PHYSIOLOGY. 

problems  of  vision,  there  are  many  others  of  a  physio-psycho- 
logical nature.  Such  for  example  are  the  visual  judgments  of 
size,  distance,  solidity,  and  color.  Judgments  of  size  and  dis- 
tance are  dependent  on:  (1)  the  size  of  the  retinal  image,  (2) 
the  effort  of  accommodation  necessary  to  obtain  sharp  defini- 
tion, and  (3)  the  amount  of  haze  which  appears  to  surround  the 
object.  Judgment  of  solidity  depends  on  the  fact  that  the 
images  produced  on  the  two  retinae  are  not  exactly  from  the 
same  point  of  view;  they  are  like  the  two  photographs  of  a 
stereoscopic  picture.  The  brain  on  receiving  these  two  slightly 
different  pictures  fuses  them  into  one,  but  judges  the  solidity  of 
the  object  from  the  differences  in  the  two  pictures. 

Judgment  of  color,  or  color  vision,  forms  a  subject  of  great 
complexity.  It  apparently  depends  on  the  existence  in  the  re- 
tina of  three  varieties  of  cones,  one  variety  for  each  of  the  three 
primary  colors.  The  primary  colors  are  red,  green  and  violet; 
and  by  mixing  them  on  the  retina  in  equal  proportions  (as  by 
rotating  a  disc  or  top  on  which  they  are  painted  as  sectors)  a 
sensation  of  white  results ;  by  using  other  proportions,  any  of  the 
other  colors  of  the  spectrum  may  be  produced.  When  one  of 
these  primary  color  receptors  is  absent  from  the  retina,  color 
blindness  exists.  Thus  if  the  red  or  the  green  receptors  are 
absent,  the  patient  cannot  distinguish  between  red  and  green 
lights.  Such  persons  cannot  be  employed  in  railway  or  nautical 
work. 


CHAPTER  XXX. 

THE  SPECIAL  SENSES  (Cont'd). 

Hearing-. 

Like  light,  sound  travels  in  waves,  but  not  as  transverse  waves 
of  the  ether  that  fills  space,  but  as  longitudinal  waves  of  con- 
densation and  rarefaction  of  the  atmosphere  itself.  The  magni- 
tude of  these  waves  is  much  greater  and  their  rate  of  trans- 
mission much  slower  than  the  waves  of  light;  therefore  we  see 
the  fiash  of  a  gun  long  before  we  hear  its  sound.  The  several 
qualities  of  sound,  such  as  pitch,  loudness  and  quality  or  timber, 
depend  respectively  on  the  frequency,  the  magnitude  and  the 
contour  of  the  waves.  Sound  waves  are  not  appreciated  by  the 
ordinary  nerve  receptors  but  only  by  those  of  the  cochlear 
division  of  the  eighth  nerve.  These  are  connected,  in  the  cochlea 
of  the  internal  ear,  with  a  highly  specialized  receptor  capable 
of  converting  the  sound  waves  into  nerve  impulses.  The  cochlea 
consists  of  a  bony  tube  wound  two  and  one-half  times  as  a  spiral 
around  a  central  column,  up  the  center  of  which  runs  the 
end  of  the  cochlear  nerve.  A  longitudinal  section  of  the 
cochlea  (Fig.  56),  therefore  shows  us  this  spiral  tube  in  sec- 
tion at  several  places,  and  it  is  noticed  that  there  projects 
into  it  from  the  central  column  a  ledge  of  bone  having  a  C-shaped 
free  margin.  From  the  lower  lip  of  the  C,  a  membrane  called 
the  basilar  membrane,  stretches  across  the  tube,  which  it  thus 
divides  into  two  canals,  of  which  the  upper  is  again  divided  into 
two  by  another  membrane  running  from  the  upper  surface  of  the 
bony  ledge. 

The  basilar  membrane  is  a  very  important  part  of  the  mechan- 
ism for  reacting  to  sound  waves.  Resting  on  it  is  a  peculiar  struc- 
ture called  the  Organ  of  Corti  (Fig.  57),  which  in  transverse  sec- 
tions of  the  cochlear  canal  is  seen  to  be  composed  of  two  rows  of 
long  epithelial  cells  set  up  on  end  like  the  rafters  of  a  roof,  with 

291 


292 


HUMAN  PHYSIOLOGY. 


shorter  ' '  hair ' '  cells  leaning  up  against  them,  particularly  on  the 
side  away  from  the  central  column.  The  sound  waves,  which  act  on 
the  basilar  membrane,  are  transmitted  to  the  fluid,  which  fills  the 
uppermost  of  the  three  divisions  of  the  cochlear  tube  (see  Fig. 
56),  through  a  membrane  covering  an  oval-shaped  opening  (the 
oval  window)  in  the  bony  partition  separating  the  internal  from 
the  middle  ear.  After  reaching  the  apex  of  the  cochlea  they  pass 
through  a  small  aperture  in  the  basilar  membrane  into  the  lowest 


Fig.  56. — Semidiagrammatic  section  through  the  right  ear  (Czermak)  :  G, 
external  auditory  meatus ;  T,  membrana  tympani ;  P,  tj^mpanic  cavity  or 
middle  ear  with  the  auditory  ossicles  stretching  across  it  and  the  Eustachian 
tube  (JS)  entering  it;  o,  oval  window;  r,  round  window;  B,  semicircular 
canals  ;  8,  cochlea  ;  Vt,  upper  canal  of  cochlea  ;  Pt,  lower  canal  of  cochlea. 
(From  Howell's  Physiology.) 


canal,  down  which  they  travel  to  lose  themselves  against  the  mem- 
brane covering  another  opening  (the  round  window)  situated 
near  the  oval  window  in  the  same  partition  of  bone.  As  they  pass 
along  these  canals  the  waves  cause  the  basilar  membrane  to  move 
or  vibrate.  The  vibration  affects  the  cells  of  the  Organ  of  Corti, 
and  so  sets  up  nerve  impulses  which  are  transmitted  to  the  coch- 
lear nerve  by  means  of  nerve  fibers  which  connect  with  each 
of  the  main  cells  of  the  Organ.    A  fine  membrane  (called  Tec- 


Fig.  57. — Diagrammatic  view  of  the  organ  of  Corti  (Testut)  :  D.  basilar 
membrane :  A,  B.  inner  and  outer  rods  of  Corti ;  «,  6',  G,"  hair  cells ;  7,  T, 
supporting  cflls.       (From    Howell's    Physiology.) 


HEARING.  293 

tonal)  rests  on  the  tops  of  the  hair  cells,  and  by  rubbing  on  them 
when  they  move,  this  membrane  augments  the  action  of  the 
basilar  membrane. 

We  must  now  consider  how  the  sound  waves  are  brought 
from  the  outside  to  the  oval  window.  The  pinna  of  the  ear  col- 
lects the  sound  waves  from  the  outside  and  directs  them  into  the 
external  auditory  canal,  at  the  inner  end  of  which  they  strike 
the  drum  of  the  ear  or  tympanic  membrane.  This  membrane  is 
stretched  loosely  in  an  oblique  direction  across  the  canal,  and  is 
composed  partly  of  fibers  which  radiate  to  the  edge  of  the 
membrane  from  the  handle  of  the  malleus,  a  process  of  one  of 
the  auditory  ossicles,  to  which  it  is  attached.  Because  of  these 
properties,  the  tympanic  membrane,  unlike  an  ordinary  drum, 
is  capable  of  vibrating  to  a  great  variety  of  notes,  and  the 
vibrations  cause  the  handle  of  the  malleus  to  move  in  and  out. 
Between  the  tympanic  membrane  and  the  cochlea  is  the  middle 
ear,  or  tympanum,  consisting  of  a  cavity  across  which  stretches 
the  auditory  ossicles  composed  of  three  small  bones,  the  malleus, 
the  incus  and  the  stapes.  Besides  the  long  process  or  handle 
already  described,  the  malleus  consists  of  a  rounded  head  sit- 
uated above  and  forming  a  saddle-shaped  articulation  with  the 
head  of  the  incus  and  a  short  process  which  runs  from  just  be- 
low the  head  to  the  anterior  M^all  of  the  tympanum.  The  incus 
is  somewhat  like  a  bicuspid  tooth,  the  malleus  articulating  with 
the  crown,  and  having  two  fangs,  a  short  one  passing  backward 
and  a  long  one  vertically  downwards.  This  process,  at  its  lower 
end,  suddenly  bends  inwards  to  form  a  ball  and  socket  joint 
with  a  stirrup-shaped  bone  (the  stapes),  the  foot  piece  of  which 
is  oval  in  shape  and  fits  into  the  oval  window  already  mentioned. 

The  ossicles  act  together  as  a  bent  lever,  the  axis  of  rotation 
passing  through  the  short  process  of  the  malleus  in  front  and  the 
short  process  of  the  incus  behind.  If  perpendiculars  be  drawn 
from  this  axis  to  the  tips  of  the  handle  of  the  malleus  and  the 
long  process  of  the  incus,  i\  will  be  found  that  the  latter  is  only 
two-thirds  the  length  of  the  former  (Fig.  58).  The  amplitude 
of  movement  at  the  stapes  will  therefore  be  only  two-thirds  of 
that  at  the  center  of  the  tympanic  membrane,  but  one  and  one- 


294 


HUMAN   PHYSIOLOGY. 


half  times  stronger.  The  increase  in  force  with  which  the 
movements  of  the  tympanic  membrane  are  conveyed  to  the  oval 
window  is  still  further  magnified  by  the  fact  that  the  latter  is 
only  one-twentieth  the  size  of  the  former.  It  is  by  these  move- 
ments at  the  oval  window  that  waves  are  set  up  in  the  fluid 
occupying  the  uppermost  membranous  tube  of  the  cochlea  and 
thus  acting  on  the  basilar  membrane.     The  tympanic  cavity  or 


Fig.  58. — Tympanum  of  right  side  with  the  auditory  ossicles  in  place  (Mor- 
ris) :  1,  incus  (like  bicuspid  tooth)  with  one  process  (.?)  attached  to  wall  of 
tympanum  and  the  other  running  downwards  to  articulate  at  9  and  8,  the 
stapes;  16,  head  of  malleus  attached  to  tympanic  membrane.  (From  How- 
ell's Physiology.) 


tympanum  across  which  the  chain  of  ossicles  stretches  is  kept 
at  atmospheric  pressure  by  the  Eustacliian  tube,  which  connects 
it  with  the  posterior  nares. 

Deafness  may  he  due  to  tlie  folloiving  causes: 

1.  Rupture  of  the  tympanic  membrane. 

2.  Ankylosis  or  stiffening  of  the  joints  between  the  ossicles 


THE    SENSE    OF    TASTE.  295 

and  the  ligaments  which  hold  them  in  place  in  the  tympanic 
cavity.  Flexibility  of  the  joints  between  the  ossicles  prevents 
sudden  jars  at  the  oval  window,  for  the  joint  between  the  mal- 
leus and  incus,  being  saddle-shaped,  unlocks  whenever  abnormal 
or  excessive  movements  are  transmitted  to  the  malleus. 

3.  Blocking  of  the  Eustachian  tube.  This  is  quite  com- 
monly a  result  of  aden&ids  or  it  may  be  due  simply  to  a  catarrh 
of  the  tube.  The  result  of  the  block  is  that  the  pressure  on  the 
tympanic  cavity  falls  below  that  of  the  atmosphere  because  of 
absorption  of  oxygen  into  the  blood,  and  the  tympanic  mem- 
brane bulges  inwards  and  becomes  stretched  so  that  it  cannot 
vibrate  properly  to  the  sound  waves.  The  deafness  in  this  case 
is  easily  removed  by  reopening  the  Eustachian  tube  by  forcing 
air  into  it.  This  can  be  done  by  attaching  a  large  syringe  bulb 
to  one  nostril,  closing  the  other  nostril,  and  while  the  patient  is 
swallowing  a  mouthful  of  water,  suddenly  compressing  the  bulb. 

The  auditory  distress  which  is  experienced  by  a  person  on 
going  into  compressed  air  (as  into  a  caisson)  is  also  due  to  dis- 
turbance in  the  tympanic  pressure,  for  it  takes  a  few  moments 
before  this  reaches  that  on  the  outside.  Blowing  the  nose  usually 
removes  the  distress. 

In  all  these  conditions,  the  patient  hears  perfectly  when  a 
tuning  fork  is  applied  to  the  skull  or  teeth.  This  is  because  the 
sound  vibrations  are  then  transmitted  to  the  cochlea  through 
the  bones  of  the  head.  When  the  cochlea  is  diseased,  however, 
the  tuning  fork  cannot  be  heard  either  when  it  is  sounded  in  the 
air  or  when  it  is  applied  to  the  skull  or  teeth. 

The  Sense  of  Taste. 

Scattered  over  the  mucous  membrane  of  the  tongue  and  buccal 
cavity,  and  extending  back  into  the  pharynx  and  even  into  the 
larynx,  are  the  receptors  of  taste,  or  taste  huds.  They  are  most 
numerous  in  the  grooves  around  the  circumvallate  papillae  at 
the  root  of  the  tongue,  and  in  the  fungiform  papillas.  Each 
taste  bud  is  composed  of  a  mass  of  fusiform  cells  packed  like  a 
barrel  filled  with  staves.  The  staves  in  the  center  project  as 
hairs  beyond  those  on  the  outside,  and  it  is  evidently  by  action 


296 


HUMAN   PHYSIOLOGY. 


on  these  hairs  that  certain  dissolved  substances  set  up  a  stimulus 
of  taste.  This  stimulus  is  then  conveyed  by  fine  nerve  fibers 
which  arborize  around  the  taste  cells,  to  the  chorda  tympani  and 
lingual  nerves  in  the  anterior  portion  of  the  tongue  and  the 
glossopharyngeal  in  the  posterior  part.  Through  these  nerves 
the  sensations  are  carried  to  the  combined  afferent  nucleus  of 
the  fifth  and  ninth  nerves  in  the  medulla  oblongata  (see  Fig. 
59). 

eTros.svu(«r'|ia(il\s  imVivor 


Fig.  59. — Schema  to  show  the  course  of  the  taste  fibers  from  tongue  to 
brain  (Gushing).  The  dotted  lines  represent  the  course  as  indicated  by  Cush- 
ing's  observations.  The  full  black  lines  indicate  another  path  by  which  the 
impulses   may   reach   the   brain.      (From   Howell's    Physiology.) 


Substances  cannot  be  tasted  unless  they  are  in  solution,  thus, 
quinine  powder  is  tasteless.  One  of  the  functions  of  saliva  is 
to  bring  substances  into  solution  in  order  that  they  may  be 
tasted. 

There  are  four  fundamental  taste  sensations :  sweet,  saline, 
bitter  and  sour  or  acid.  The  ability  to  distinguish  each  of  these 
tastes  is  not  evenly  distributed  over  the  tongue,  but  occurs  in 
definite  areas.     These  can  be  mapped  out  by  applying  solutions, 


THE    SENSE    OF    TASTE.  297 

possessing  one  or  another  of  these  qualities,  by  means  of  a  fine 
camel-hair  brush,  to  different  portions  of  the  tongue  previously 
dried  somewhat  with  a  towel.  Bitter  taste  is  absent  from  all 
parts  of  the  tongue  except  the  base,  hence  a  mouthful  of  a  weak 
solution  of  quinine  sulphate  has  practically  no  taste  until  it  is 
swallowed,  when  however  it  tastes  intensely  bitter.  Sweet  and 
sour  tastes  are  most  acute  at  the  tip  and  sides  of  the  tongue. 
Saline  taste  is  more  evenly  distributed. 

This  location  of  taste  sensations  is  not  a  hard  and  fast  one, 
for  neighboring  taste  buds  in,  say,  the  bitter  area  at  the  root  of 
the  tongue  may  appreciate  different  tastes;  thus,  if  a  solution 
containing  quinine  and  sugar  be  applied  to  one  papilla,  it  may 
taste  sweet,  whereas  when  applied  to  a  neighboring  one,  it  tastes 
bitter.  With  weak  solutions  one  taste  may  neutralize  another; 
thus  the  addition  of  a  small  amount  of  salt  to  a  weak  sugar  solu- 
tion may  remove  its  sweet  taste.  This  neutralization  of  one 
taste  by  another  does  not  occur  when  the  solutions  are  stronger ; 
thus  a  mixture  of  acid  and  sugar,  as  in  lemonade,  causes  stimu- 
lation of  both  "acid"  and  "sweet"  taste  buds.  The  stimula- 
tion of  one  kind  of  taste  bud  may  cause  other  taste  buds  to  be- 
come more  acutely  sensitive,  which  explains  the  sweetish  taste  of 
water  after  washing  out  the  mouth  with  a  solution  of  salt. 

Attempts  have  been  made  to  correlate  the  chemical  structure 
of  organic  substances  with  the  taste  which  thej^  excite,  but  with 
little  success.  Thus  pure  proteins  have  very  little  taste,  whereas 
half-digested  protein  is  intensely  bitter;  on  the  other  hand,  the 
pure  amino  acids,  which  form  a  large  proportion  of  the  de- 
composition products  in  such  a  digest,  are' sweet.  In  the  case  of 
acids  and  alkalies,  however,  it  has  been  established  that  the  acid 
taste  is  due  to  the  H-ion  and  the  alkaline  to  the  OH-ion.  Some 
acids,  such  as  acetic,  taste  more  acid  than  we  should  expect  from 
their  degree  of  dissociation  into  H-ions.  This  is  because  of 
their  power  of  penetration  into  the  cells  of  the  taste  buds.  When 
platinum  terminals  from  a  battery  are  applied  to  the  tongue, 
the  positive  pole  tastes  alkaline  and  the  negative  acid,  because 
OH-ions  accumulate  at  the  former  and  H-ions  at  the  latter. 

The  Assocl\tion  op  Taste,  Touch  and  Smell. — The  four 


298  HUMAN  PHYSIOLOGY. 

fundamental  tastes  do  not  nearly  represent  all  the  tastes  and 
flavors  with  which  we  are  familiar.  The  relish  of  an  appetizing 
meal,  the  piquancy  of  condiments,  the  bouquet  of  a  fine  wine, 
would  remain  unappreciated  were  there  no  other  nerve  receptors 
than  those  described  above.  Two  other  types  of  nerve, receptors 
are  involved,  namely,  (1)  those  of  common  sensation,  as  in  the 
case  of  acids,  which  add  an  astringent  character  to  the  sour 
taste,  and,  (2)  those  of  smell,  as  in  wines  and  flavored  foods. 
The  importance  of  the  sense  of  smell  in  '' tasting"  explains  the 
loss  of  this  ability  during  nasal  catarrh  or  cold  in  the  head. 
Under  such  conditions  an  apple  and  an  onion  may  taste  alike. 
Certain  drugs  when  applied  to  the  tongue  affect  taste  sensa- 
tions in  different  degrees.  Thus  cocaine  first  of  all  paralyzes 
the  receptors  of  common  sensation  so  that  pain  is  no  longer 
felt  and  an  acid  loses  all  of  its  astringent  qualities  and  merely 
tastes  sour.  A  little  later  the  bitter  taste  also  disappears,  then 
salt,  then  sour,  but  the  saline  taste  remains  even  after  the  cocaine 
has  developed  its  full  effect.  Another  interesting  drug  acting 
on  the  taste  sensations,  is  a  substance  present  in  the  leaves  of 
Gymnema  sylvestre.  When  these  leaves  are  chewed,  the  sweet 
and  bitter  tastes  are  absent,  those  of  acid  and  of  salt  and  ordi- 
nary sensation    (astringency,   etc.)    being,  however,  unaffected. 

The  Sense  of  Smell. 

In  man  the  sense  of  smell  is  very  feeble  when  compared  with 
that  of  the  lower  animals,  and  it  is  of  very  unequal  development 
in  different  individuals.  It  is,  moreover,  readily  fatigued,  as  is  the 
experience  of  every  one  who  has  been  compelled  to  live  in  stuffy 
rooms.  The  receptors  are  represented  by  the  columnar  epithelium 
of  the  superior  and  middle  turbinate  bones  and  the  adjacent 
parts  of  the  nasal  septum.  This  epithelium  is  composed  of  large 
columnar  cells,  each  cell  being  connected  with  a  nerve  fiber  which 
is  one  of  the  branches  of  a  fusiform  bipolar  nerve  cell  lying  im- 
mediately beneath  the  epithelium.  The  second  branch  of  each 
nerve  cell  runs  through  the  cribiform  plate  to  join  the  olfactory 
bulb.  After  making  connections  with  nerve  cells  here,  the  path- 
way is  continued  along  the  olfactory  tract  to  the  hippocampal 


THE    SENSE   OF    SMELL.  299 

region  of  the  brain.  As  we  would  expect,  this  portion  of  the 
brain  is  highly  developed  in  those  animals  having  a  very  acute 
sense  of  smell. 

The  olfactory  epithelium  is  kept  constantly  moist  with  fluid, 
and  substances  cannot  be  smelled  unless  the  odorous  particles 
which  they  give  off  become  dissolved  in  this  fluid.  These  odor- 
ous particles  diffuse  into  the  upper  nares  from  the  air  currents 
which,  with  each  respiration,  are  passing  backwards  and  for- 
wards along  the  lower  nasal  passages.  There  is  no  actual  move- 
ment of  air  over  the  olfactory  epithelium. 

Nature  of  Stimulus. — It  is  impossible  to  state  just  exactly 
what  it  is  that  emanates  from  an  odorous  body  to  excite  the  ol- 
factory sense.  All  we  can  say  is  that  it  does  not  require  to  be 
present  in  more  than  the  merest  trace  in  the  air  in  order  to  un- 
fold its  action.  Thus  even  in  the  case  of  man,  with  his  undevel- 
oped sense  of  smell,  0.000,000,000,04  of  a  gramme  of  mercaptan, 
suspended  in  a  liter  of  air,  can  be  smelled,  and  in  the  case  of  the 
dog,  the  dilution  may  no  doubt  be  many  thousand  times  greater. 
The  sense  of  smell  is  the  most  important  of  the  projicient  sensa- 
tions in  certain  aquatic  animals,  and  is  very  closely  associated 
with  the  sexual  functions  of  the  animal.  Just  as  in  the  case  of 
taste,  certain  substances  owe  their  peculiar  odors  to  simultane- 
ous stimulation  of  the  olfactory  epithelium  and  the  receptors  of 
common  sensation.  Thus  the  pungency  of  acids,  of  ammonia, 
chlorine,  etc.,  is  due  to  stimulation  of  the  endings  of  the  fifth 
nerve.  Attempts  have  been  made  to  classify  odors,  as  has  been 
done  for  tastes,  but  with  no  success. 


CHAPTER  XXXI. 
THE  MUSCULAR  SYSTEM. 

The  General  Properties  of  Muscular  Tissues. — The  intimate 
nature  of  the  physical  changes  taking  place  during  the  contrac- 
tion of  a  muscle  are  not  understood,  and  the  histological  changes 
which  occur  have  had  various  interpretations  put  on  them.  For 
a  discussion  of  these  a  textbook  on  histology  should  be  consulted. 

The  physiological  property  which  distinguishes  muscular  tis- 
sue from  other  forms  of  tissue  is  that  of  contractility.  It  is  to 
this  property  that  the  forcible  shortening  of  the  muscles  which 
produces  movements  is  due.  The  shortening  occurs  in  the  long 
axis  of  the  muscle  and  is  accompanied  by  a  compensatory  thick- 
ening in  the  transverse  diameter,  which  keeps  the  bulk  of  the 
muscle  constant.  After  the  period  of  active  contraction  the 
muscle  remains  in  the  contracted  position  unless  it  be  pulled  back 
into  extension  by  some  force.  No  isolated  muscle  can  actively 
expand ;  it  can  only  do  so  passively.  Muscle  does  not  possess  the 
property  of  initiating  the  contraction.  This  depends  on  the  ner- 
vous system  acting  on  another  property  of  muscle,  namely,  its 
irritability ,  that  is,  the  ability  of  the  muscle  to  react  very  quickly 
to  a  stimulus.  The  amount  of  stimulus  which  it  requires  is  very 
small  compared  with  the  reaction  brought  about  in  the  muscle. 

A  muscle  can  be  stimulated  in  other  ways  than  through  its 
nerve,  namely,  by  mechanical,  thermal,  electrical,  and  chemical 
stimuli  applied  directly  to  it.  By  using  these  artificial  stimuli 
on  muscles  excised  from  the  body  the  properties  of  muscular 
contraction  can  be  studied. 

A  record  of  the  contraction  of  a  muscle  of  a  frog  may  be  made 
by  excising  it  and  attaching. one  end  to  a  suitable  clamp  and  the 
other  end  to  a  light  lever  the  opposite  end  of  which  is  arranged 
to  trace  on  smoked  paper  placed  on  a  rapidly  revolving  drum. 
If  such  a  muscle  be  electrically  excited,  it  will  record  its-  con- 
traction as  a  curve  on  the  smoked  surface  of  the  paper,  and  show 

300 


THE  MUSCULAR  SYSTEM.  301 

a  number  of  interesting  details  as  to  the  properties  of  contracting 
muscles. 

The  muscle  does  not  begin  to  contract  at  the  exact  moment  that 
the  stimulus  is  applied.  A  very  short  latent  period  (.01  sec.) 
elapses  between  the  stimulus  and  the  beginning  of  the  contrac- 
tion. During  this  time  the  muscle  is  undergoing  some  internal 
change  which  must  precede  the  contraction.  The  period  of  active 
contraction  is  relatively  short  (.04  sec.)  and  the  period  of  relax- 
ation somewhat  longer  (.05  sec).  The  ordinary  movements  of 
the  body  cannot  obviously  be  of  the  nature  of  a  single  muscular 
contraction,  for  they  much  exceed  one-tenth  of  a  second  in  dura- 
tion. They  are  in  fact  produced  by  a  prolonged  contraction  of 
muscles  caused  by  the  fusions  of  several  single  contractions.  This 
is  known  as  tetanic  cont7'actio7i,  smd  it  can  easily  be  produced 
in  the  muscle  preparation  described  above  by  giving  it  a  series 
of  electrical  stimuli  from  an  induction  coil.  If  the  stimuli 
be  properly  timed,  a  contraction  curve  somewhat  higher  and 
showing  no  relaxation  phase  will  be  produced.  When  the  ex- 
citation is  discontinued,  the  muscle  returns  to  its  normal  length. 

The  amount  of  load  which  the  muscle  lifts  has  a  peculiar  effect. 
Up  to  a  certain  point  an  increase  in  the  load  increases  the  effi- 
ciency of  the  muscle  and  the  muscle  will  actually  perform  more 
work  with  a  moderate  load  than  with  no  load  at  all.  After  a 
certain  load  is  reached,  the  efficiency  of  the  muscle  begins  to 
diminish  and  further  increase  of  the  load  decreases  the  work 
accomplished  by  the  muscle.  The  principle  involved  here  is  made 
use  of  by  fork  and  shovel  manufacturers,  who  are  careful  to 
make  their  implements  carry  the  load  best  suited  to  develop  the 
maximal  efficiency  of  the  muscles  of  a  normal  average  man.  Al- 
lowing the  laborer  to  choose  his  own  shovel  is  not  always  the 
best  for  the  laborer  or  for  his  employer. 

Another  interesting  fact  is  that  a  contracted  muscle  is  more 
elastic  than  a  relaxed  muscle.  Equal  weights  attached  to  a  con- 
tracted and  to  a  relaxed  muscle  will  produce  a  greater  elonga- 
tion in  the  contracted  than  in  the  relaxed  muscle.  It  is  this  prop- 
erty which  protects  the  muscle  from  sudden  rupture  when  at- 
tempts are  made  to  lift  loads  that  are  too  heavy. 


302  HUMAN  PHYSIOLOGY. 

The  Chemical  Changes  Which  Accompany  Muscular  Contrac- 
tion are  concerned  in  the  liberation  of  energy  by  the  oxidation 
of  the  organic  foodstuffs  and  the  converting  of  this  energy  into 
museular  energy.  Just  how  this  change  is  brought  about  is  not 
known.  During  muscular  activity  a  great  amount  of  oxygen  is 
required  and  a  large  amount  of  carbon  dioxide  is  given  off.  It 
is  very  interesting,  however,  to  know  that  the  maximal  exchange 
of  these  gases  does  not  actually  accompany  but  follows  the  mus- 
cular activity,  thus  indicating  that  a  muscle  becomes  charged 
with  energy,  so  to  speak,  during  rest  and  discharges  itself  in 
much  the  same  manner  as  a  storage  battery  during  a  period  of 
activity.  If  a  muscle  be  made  to  contract  till  it  becomes  fatigued, 
a  large  amount  of  sarco-lactic  acid  accumulates  in  the  tissue.  This 
poisons  the  muscle  and  makes  it  unable  to  contract.  If  this  be 
washed  out  with  saline,  the  muscle  will  again  contract  for  a  time. 
Rigor  mortis,  or  the  rigidity  which  comes  on  after  death,  may 
be  due  to  the  development  of  sarco-lactic  acid  in  the  tissues  be- 
cause they  have  become  deprived  of  oxygen. 


CHAPTER  XXXII. 
REPRODUCTION. 

The  most  important  function  of  an  animal 's  life  is  the  produc- 
tion of  a  new  individual  which  in  all  peculiarities  of  function  and 
structure  is  essentially  like  the  parent.  The  fundamental  prob- 
lems of  the  process  of  reproduction  which  are  of  physiological 
importance,  are  those  of  fertilization  and  heredity.  Fertiliza 
tion  consists  in  the  union  of  two  parent  cells  to  produce  a  new 
cell  which  is  endowed  with  the  power  of  growth  and  subdivision. 
Heredity  refers  to  the  phenomenon  which  directs  the  cell  thus 
fertilized  to  develop  into  an  individual  like  its  parents. 

Since  up  to  the  present  time  most  of  our  knowledge  of  these 
processes  is  based  on  anatomical  data,  we  will  discuss  them  very 
briefly  and  will  pay  more  attention  to  what  we  may  term  the 
accessory  phenomena  of  reproduction,  which  are  of  more  practi- 
cal interest  at  present. 

Reproduction  in  the  unicellular  animals  is  a  simple  process. 
The  parent  cell  divides  exactly  in  halves  and  two  daughter  cells 
are  produced.  In  the  multicellular  animals  this  type  of  repro- 
duction is  impossible  and  the  process  is  delegated  to  a  portion 
of  the  animal's  body  known  as  the  reproductive  system.  This 
system  in  man  includes  the  specialized  tissues  which  produce  the 
cells  or  eggs  from  which  the  new  individual  develops,  and  the 
accessory  organs  which  are  concerned  in  providing  favorable 
conditions  for  the  development  of  these  cells. 

Fertilization. — A  very  simple  type  of  fertilization  is  seen  in 
unicellular  animals,  which  ordinarily  reproduce  by  simple  divi- 
sion. After  a  series  of  simple  divisions  the  cell  becomes  unable 
to  develop  more  cells  until  after  it  has  united  with  another  cell 
to  form  one  large  cell.  This  process  is  termed  conjugation.  In 
higher  forms,  the  development  of  the  egg  is  always  preceded  by 
the  phenomenon  of  fertilization,  which  is  somewhat  similar  to 

303 


304  HUMAN  PHYSIOLOGY. 

that  of  conjugation  in  lower  forms.  In  this  process,  cells  of  two 
types  are  concerned,  the  male,  or  sperm  cell,  or  spermatozoon, 
and  the  female  cell  or  ovum.  The  spermatozoon  has  the  ability 
to  move  and  to  penetrate  the  ovum.  The  nuclear  elements  of 
both  cells  unite  to  form  a  new  nucleus,  which  is  then  capable  of 
undergoing  a  long  series  of  subdivisions.  In  changes  which  pre- 
cede fertilization,  the  nuclear  material  originally  present  in  both 
male  and  female  cells  is  reduced,  and  when  the  cells  fuse,  the  re- 
sulting nucleus  contains  a  normal  quantity  of  nuclear  material. 

The    Accessory    Phenomena    of    Reproduction    in    Man. — 

The  beginning  of  the  active  sexual  life  in  man  is  between  the 
ages  of  fourteen  and  sixteen,  and  is  called  the  age  of  puberty. 
In  both  boys  and  girls  the  whole  body  shows  a  marked  develop- 
ment at  this  time.  The  growth  of  hair  on  the  pubic  regions  and 
arm  pits,  and  on  the  face  of  boys,  the  deepening  of  the  male 
voice,  and  the  development  of  the  breasts  in  the  female,  are  all 
accompanying  phenomena  of  the  development  of  puberty.  In 
females  this  age  is  marked  by  the  onset  of  menstruation,  which 
consists  of  a  periodic  flow  of  mucus  and  blood  from  the  uterus. 
The  flow  lasts  from  four  to  five  days,  and  recurs  with  great  regu- 
larity about  every  four  weeks.  In  males  fully  formed  seminal 
fluid,  containing  live  sperm  cells,  appears. 

The  Female  Organs  of  Reproduction. — These  are  the  ovaries, 
oviducts,  uterus  and  the  vagina.  The  ovaries  are  paired  bodies 
lying  in  the  lower  part  of  the  abdominal  cavity  and  held  in  posi- 
tion by  the  broad  ligament.  The  cells  from  which  the  ova  de- 
velop are  imbedded  in  the  fibrous  tissue  of  the  ovary.  A  number 
of  these  cells,  better  developed  than  their  fellows,  and  surrounded 
by  a  layer  of  cells,  which  form  a  sort  of  follicle,  lie  near  the  sur- 
face of  the  ovary.  These  are  the  Graafian  follicles,  in  which  the 
ova  develop  till  they  are  ripe,  when  they  are  extruded  into  the 
abdominal  cavity  by  rupture  of  the  follicle.  In  very  close  appo- 
sition to  the  ovaries  is  a  tube,  the  oviduct,  which  leads  to  the 
uterus.  The  outer  end  of  this  tube  is  fimbriated,  and  it  is  fur- 
nished with  cilia,  the  movements  of  which  cause  currents  in  the 
fluids  of  the  abdominal  cavity,  and  which    direct   the    ova   dis- 


REPRODUCTION.  305 

chargf^d  from  the  follicle  into  the  oviduct.  The  uterus  is  a  pear- 
shaped  organ  with  muscular  walls.  It  is  about  7  cm.  in  length, 
and  consists  of  an  upper  dilated  portion,  called  the  fundus,  and 
a  lower  constricted  portion,  called  the  cervix.  The  cervix  opens 
by  a  small  aperture  into  the  vagina,  which  is  a  membranous  canal 
about  10  cm.  long  extending  to  the  vaginal  outlet  at  the  external 
genitalia. 

The  Male  Organs  of  Generation  are  the  testicles,  vas  deferens, 
seminal  vesicles,  the  penis,  the  prostate  gland,  and  a  number  of 
small  glands  along  the  uretlira. 

The  testicles  consist  of  two  parts,  a  portion  of  which  is  cellular 
and  is  concerned  in  the  development  of  the  spermatozoa;  and  a 
portion  called  the  epididymis,  containing  the  lower  portion  of 
the  very  long  and  convoluted  duct,  the  vas  deferens.  This  duct 
connect.'  the  testicles  with  the  seminal  vesicles,  which  lie  at  the 
base  of  lie  bladder  and  in  close  relation  to  the  prostate  gland. 
The  seminal  vesicles  are  united  by  a  short  duct  with  the  urethra, 
which  is  the  outlet  for  the  excretions  of  both  the  kidney  and  the 
testicles. 

The  spermatozoa  are  developed  in  the  testicles  and  find  their 
way  to  the  seminal  vesicles  through  the  vas  deferens.  On  their 
way  they  become  mixed  with  a  number  of  fluid  secretions,  the 
chief  of  which  are  derived  from  the  seminal  vesicles  of  the  pros- 
tate gland  and  of  the  glands  of  Cowper.  The  resulting  mixture 
is  the  seminal  fluid. 

Impregnation. — The  seminal  fluid  containing  the  spermatozoa 
is  deposited  in  the  vagina  during  coitus.  Attracted  by  the  acid 
reaction  of  the  secretions  of  the  uterus  or  under  an  unknown  in- 
fluence, the  spermatozoa  soon  enter  the  uterine  cavity  through 
its  opening  into  the  vagina,  and  find  their  way  to  the  oviduct, 
where  they  remain  waiting  for  the  ovum  to  appear. 

Ovulation. — At  about  the  time  of  a  menstrual  period  an  ovum 
.  is  discharged  from  a  ripened  Graafian  follicle  and  finds  its  way 
into  the  oviduct  by  way  of  the  fimbriated  extremity  of.  the  tube, 
down  which  it  is  conducted  to  the  uterus.  It  is  a  debated  ques- 
tion as  to  wljat  the  exact  relation  between  menstruation  and  ovu- 


306  HUMAN  PHYSIOLOGY. 

lation  may  be.  Whether  ovulation  precedes  or  follows  menstrua- 
tion is  not  known,  but  the  weight  of  evidence  favors  the  belief 
that  menstruation  serves  to  prepare  the  uterine  walls  for  the 
reception  of  the  fertilized  ovum  should  one  be  discharged.  In 
animals  there  are  periods,  called  the  rutting  period,  during 
which  impregnation  of  the  ovum  with  the  spermatozoon  is  pos- 
sible. Preceding  this  period  there  occurs  a  swelling  of  the  exter- 
nal genitalia  and  some  discharge  of  mucus.  This  period  probably 
corresponds  to  the  menstrual  period  in  woman,  for  there  is  much 
evidence  to  show  that  impregnation  occurs  most  frequently  fol- 
lowing the  menses. 

Menstruation  ceases  during  pregnancy  and  is  generally  absent 
during  the  period  of  lactation.  It  ceases  altogether  between  the 
ages  of  about  forty-five  and  fifty.  After  this  time,  which  is 
known  as  the  climacteric  period,  a  woman  is  no  longer  capable 
of  bearing  children. 

The  union  of  the  spermatozoon  and  the  ovum  usually  occurs 
in  the  oviduct.  If  the  ovum  is  not  fertilized  it  is  cast  off.  If  it 
is  fertilized,  a  considerable  thickening  of  the  uterine  mucous 
membrane  takes  place  from  the  proliferation  of  its  cells.  When 
the  ovum  reaches  the  uterus,  it  becomes  imbedded  in  the  mucous 
membrane  of  the  fundus  of  the  uterus.  This  mucous  membrane 
is  very  vascular  and  soon  becomes  fused  with  the  outer  layer  of 
the  ovum. 

Pregnancy. — At  first  the  ovum  receives  its  nourishment 
directly  from  the  mucous  membrane  of  the  uterus,  but  as  the 
ovum  develops  and  becomes  what  we  term  an  embryo,  the  part 
lying  next  to  the  uterine  mucosa  becomes  very  vascular ;  a  similar 
process  takes  place  in  the  uterine  mucosa  directly  in  contact  with 
the  embryo.  By  this  process  the  placenta  is  formed,  the  organ 
through  which  the  embryo  obtains  nourishment  from  the  mother. 

The  vascular  system  of  the  embryo  is,  however,  entirely  sepa- 
rate from  the  maternal  vessels,  and  the  blood  of  the  mother 
never  directly  enters  the  embryo.  The  interchange  between  the 
two  must  be  effected  through  the  cells  covering  the  vessels  of  the 
uterine  and  foetal  portions  of  the  placenta.  In  other  words,  the 
embryo  may  be  said  to  live  a  parasitic  yet  entirely  independent 


REPRODUCTION.  307 

life,  since  through  its  placental  vessels  it  exchanges  its  effete 
products  for  the  oxygen  and  nourishment  contained  in  the 
mother's  blood. 

Birth. — "While  the  ovum  is  being  developed  into  a  human 
being  bj  division  of  the  original  cell  of  the  fertilized  ovum,  the 
uterus  becomes  very  much  enlarged,  and  its  walls  increase  in  size 
by  the  growth  of  muscular  tissue.  At  the  end  of  approximately 
280  days  from  tlte  date  of  impregnation  of  the  ovum,  the  devel- 
opment is  complete  and  birth  takes  place.  This  consists  in  the 
expulsion  of  the  foetus  by  muscular  contractions  of  the  uterus. 

Directly  the  child  is  born,  the  placenta  begins  to  separate  from 
the  uterine  wall  and  is  soon  expelled.  The  child  deprived  of  its 
placental  nourishment  must  now  begin  an  independent  life.  It 
must  take  in  its  own  oxygen  and  give  off  carbon  dioxide  by  its 
respiratory  organs.  It  must  take  its  food  through  the  alimentary 
canal,  and  excrete  its  waste  products  through  its  kidneys. 


APPENDIX. 


Since  the  amount  of  time  devoted  to  laboratory  instruction  and  the 
general  equipment  of  the  physiological  laboratories  in  our  dental 
schools  varies  greatly,  detailed  laboratory  outlines  for  an  experimental 
course  in  physiology  are  out  of  place.  The  following  is  a  summary  of 
the  experiments  in  experimental  physiology  which  is  given  at  the  pres- 
ent time  to  the  dental  classes  in  the  University  of  Illinois.  It  is  pre- 
sented here  in  hope  that  it  may  help  others  in  planning  a  course  of  like 
character.  The  exercises  occupy  thirty  periods  of  about  two  hours  each. 
"When  possible,  the  experiments  are  done  by  the  individual  students; 
the  more  diflBcult  experiments  are  given  as  demonstrations. 

EXERCISE   No.   1. 

This  period  is  devoted  to  a  study  of  the  chemical  nature  of  the  pro- 
teins. The  solubility  of  egg  white  in  distilled  water  and  in  salt  solu- 
tion is  tested.  The  coagulation  of  albumin  by  heat,  and  its  precipita- 
tion by  the  addition  of  neutral  salts,  is  shown.  The  common  chemical 
tests  for  the  proteins  are  made.  The  alterations  which  take  place  in 
the  chemical  and  physical  properties  of  protein  when  partially  digested 
are  illustrated  by  doing  the  above  experiments  on  solutions  of  peptone 
in  place  of  egg  white. 

EXERCISE   No.  2. 

The  physical  and  chemical  properties  of  the  fats,  carbohydrates  and 
inorganic  salts  found  in  the  body,  are  studied. 

EXERCISE   No.  3. 

The  physical  chemical  properties  of  water  which  are  of  interest  from 
a  physiological  standpoint,  as  for  example,  its  solvent  action,  specific 
heat,  and  latent  heat  of  vaporization,  are  compared  with  the  same  prop- 
erties of  other  solvents,  such  as  oil,  ether,  acid  and  alkali. 

EXERCISE   No.  4. 

This  consists  of  a  demonstration  of  experiments  which  illustrate  the 
phenomena  of  surface  tension,  osmosis,  dialysis  and  ionization. 

EXERCISE   No.  5. 

The  well  known  experiment  showing  the  nervous  mechanism  involved 
in  the  secretion  of  the  saliva  is  made  upon  a  dog. 

309 


310  HUMAN  PHYSIOLOGY. 

EXERCISE   No.  6. 

In  this  exercise  each  student  tests  his  own  saliva  for  its  organic  and 
inorganic  constituents.  The  digestive  power  of  the  saliva  is  estimated 
by  determining  the  length  of  time  that  it  takes  to  change  a  solution  of 
starch  into  sugar.  The  determination  of  the  neutralizing  power  of  the 
saliva  is  determined  by  Marshall's  method   (see  page  49). 

EXERCISE   No.  7. 

The  acidity  and  the  digestive  power  of  gastric  juice  are  determined 
in  this  exercise.  As  gastric  juice  is  difficult  to  obtain,  a  very  good  arti- 
ficial juice  can  be  prepared  by  dissolving  commercial  pepsin  in  0.4% 
hydrochloric  acid. 

EXERCISE   No.  8. 

The  peristaltic  muscular  movements  of  the  stomach  are  studied  by 
means  of  a  strip  of  muscle  obtained  from  the  pyloric  end  of  a  frog's 
stomach.  The  muscle  strip  is  attached  to  the  end  of  a  light  lever  which 
traces  on  the  smoked  drum  of  a  kymographion. 

EXERCISE   No.  9. 

The  secretion  of  pancreatic  juice  in  a  dog  following  the  intravenous 
injection  of  the  harmone,  secretin,  is  demonstrated.  The  pendulous  and 
peristaltic  movements  of  the  small  intestines  are  also  observed. 

EXERCISE   No.   10. 

Each  student  determines  the  number  of  red  blood  corpuscles  per 
cubic  millimeter  of  blood  in  another  student's  blood,  by  use  of  the 
hsemocytometer. 

EXERCISE   No.   11. 

The  white  sells  in  each  cubic  millimeter  of  blood  are  determined  as 
in  the  case  of  the  red  cells. 

EXERCISE   No.   12. 

The  number  of  white  cells  in  a  cubic  millimeter  of  blood  obtained 
from  patients  suffering  from  infections  of  the  mouth,  is  determined. 

EXERCISE   No.   13. 

This  period  is  used  for  demonstrating  the  physical  and  chemical  phe- 
nomena of  the  coagulation  of  the  blood. 

EXERCISE   No.   14. 

The  circulation  of  the  blood  in  the  mesentery  of  the  frog's  intestines 
is  studied  under  the  microscope. 

EXERCISE   No.   15. 

The  principles  of  the  circulation  as  shown  by  the  Lombard's  Heart 
Model  are  demonstrated. 


APPENDIX.  311 

EXERCISE    No.   16. 
This  period  is  devoted  to  a  study  of  the  physiological  properties  of 
the  turtle's  heart  muscle.     A  record  of  the  heart  beat  is  obtained  by 
attaching  the  heart  to  a  light  lever  which  records  its  movements  on 
the  revolving  drum  of  a  kymographion. 

EXERCISE    No.   17. 

A  demonstration  upon  a  dog,  showing  the  factors  which  maintain  the 
blood  pressure,  occupies  this  period.  Tracings  are  made  of  the  blood 
pressure  and  the  heart  beat  by  means  of  the  mercury  manometer  and 
the  kymographion. 

Attention  is  called  to  the  part  which  the  heart  beat  and  the  peri- 
phereal  resistance  of  the  blood  vessels  play  in  the  maintaining  of  the 
circulation  of  the  blood. 

EXERCISE  No.  18. 
The  nervous  regulation  of  the  heart  beat  is  graphically  shown  by  de- 
termining the  effect  which  section  and  subsequent  stimulation  of  the 
peripheral  ends  of  the  sympathetic  fibers  to  the  heart,  and  the  peri- 
pheral end  of  the  vagus  nerves  have  upon  the  heart  beat  and  the  blood 
pressure.  The  method  of  recording  these  observations  is  the  same  as 
in  the  previous  experiment. 

EXERCISE   No.   19. 

The  vaso-constrictor  fibers  to  the  ear  in  the  cervical  sympathetic 
nerve  of  the  rabbit  are  demonstrated.  In  the  same  animal  the  action 
of  the  cardiac  depressor  nerve  upon  the  heart  beat  and  the  blood  pres- 
sure is  sh6wn. 

EXERCISE   No.  20. 

The  effect  of  sensory  nerve  stimulation  upon  the  respiration  and  cir- 
culation is  studied.  The  central  end  of  the  fifth  nerve  in  an  anaesthe- 
tized dog  is  stimulated  with  an  electric  current,  while  tracings  of  the 
heart  beat,  blood  pressure,  and  the  respiration  are  being  taken  upon  a 
kymographion.  Likewise  the  effect  of  stimulating  the  sensory  fibers  of 
the  sciatic  nerve  (by  burning  the  paw  of  a  deeply  anaesthetized  dog) 
upon  the  blood  pressure,  etc.,  can  be  shown.  The  effects  of  asphyxia, 
haemorrhage  and  gravity  on  the  circulation  and  respiration  is  also  dem- 
onstrated. 

EXERCISE   No.  21. 

In  this  experiment  the  blood  pressure  is  recorded  as  in  the  former 
experiments.  The  changes  in  the  volume  of  the  kidney  are  determined 
by  means  of  the  oncometer.  The  effect  of  stimulation  of  the  splanchnic 
nerve  upon  the  kidney  volume  and  the  blood  pressure  and  the  effect 
which  follows  the  injection  of  adrenalin,  are  compared. 


312  HUMAN  PHYSIOLOGY. 

EXERCISE   No.  22. 

The  students,  using  each  other  as  subjects,  determine  the  tidal,  com- 
plemental,  and  supplemental  air  and  the  vital  capacity  of  their  lungs 
by  means  of  the  spirometer.  The  various  forms  of  artificial  respira- 
tion are  tested  out  and  the  most  effective  type  determined  by  compar- 
ing the  respiratory  exchange  as  measured  by  the  spirometer  or  a  gas 
meter.    A  graphic  record  of  the  respiratory  movements  is  made. 

EXERCISE   No.  23. 

An  analysis  of  the  atmospheric  and  expired  air  is  made  and  compared. 

EXERCISE   No.  24. 

The  blood  pressure  of  each  student  is  determined  by  the  ausculatory 
and  palpation  methods.  Both  systolic  and  diastolic  pressures  are  taken. 
The  effect  upon  the  blood  pressure,  of  running  up  and  down  stairs,  is 
determined. 

EXERCISE   No.  25. 

The  student  is  taught  to  recognize  the  heart  and  the  respiratory 
sounds  by  the  use  of  the  stethoscope. 

EXERCISE   No.  26. 

The  effect  of  the  administration  of  nitrous  oxide  upon  the  blood  pres- 
sure and  heart  beat  is  demonstrated.  The  various  stages  of  the  anaes- 
thesia are  indicated.  The  experiment  is  terminated  by  showing  the 
effect  upon  the  blood  pressure  and  the  heart  of  injecting  a  small  and 
a  large  dose  of  cocain  into  the  animal. 

EXERCISE   No.  27. 

This  period  is  devoted  to  the  study  of  the  physical  and  chemical  prop- 
erties of  normal  and  pathological  urine. 

EXERCISE   No.  28. 

This  is  a  demonstration  experiment  of  the  physiological  properties  of 
the  skeletal  or  voluntary  muscle. 

EXERCISE   No.  29. 

This  exercise  is  a  demonstration  of  the  reflexes  which  can  be  elicited 
in  the  spinal  frog.  The  common  reflexes  which  can  be  elicited  in  man 
are  shown  and  their  physiological  significance  pointed  out. 

EXERCISE   No.  30. 

This  period  is  devoted  to  some  of  the  simple  experiments  on  the  spe- 
cial senses. 


INDEX. 


Abducens  or  sixth  nerve,   261 
Aberration,    cliromatic,    285 

spherical,  285 
Absorption,   80 

Accelerator  nerves  of  heart,  184 
Accommodation,    281 

mechanism,  283 

pupil    in,    284 
Acidity,  30 

of  gastric  juice,   64 

of  saliva,  48 
Acromegaly,    131 
Addison's    disease    and    adrenals, 

129 
Adrenalin,    130 
Adrenals    (suprarenal   capsules), 

129 
Adsorption,  33 
Afferent  nerve  paths,  245 
Albumin,    22 
Albuminuria,  232 
Amino  bodies,  22 
Amoeba,   18 
Ammonia,   108 

in   urine,   230 
Amylopsin,  74 
Anesthesia,   245 
Analgesia,  245 
Anaphylaxis,   151 
Animal  heat.  134 
Antibodies  in  blood,  148 
Antienzymes,   36,   77 
Antipyretics,  138 
Autithrombin,  148 
Antitoxin,  130 
Apex  beat  of  heart,   162 
Aphasia,    273 
Appetite,   43,   60 
Arterial  blood  pressure,  173 
Asphyxia,  195 

Assimilation  (see  Metabolism) 
Association    areas   of    cerebrum, 
272 

fibers  of  cerebrum,  272 
Associative  memory,  273 
Asthma,  222 


Astigmatism,    286 
Atmosphere  and  metabolism,  88 
Auditory  areas  of  cerebrum,  272 
Auditory  ossicles,  293 
Augmentor  nerves  of  heart,  184 
Auricle,   167 

Auriculo-ventricular    valves,    159 
Auscultation  of  lungs,  213 
Autonomic  nervous    system,   277 

Bacterial  digestion,  66,  76 
Beat  of  heart,  161 
Beef  tea,   107 
Beri-Beri,  121 
Bile,  71 

Binocular  vision,   289 
Bladder,   urinary,   235 
Blind   spot,   288 
Blood,    140 

coagulation  of,  147 

functions  of,  140 

gases   of,   201 

microscopic  characters  of,  140 

physical  properties  of,  140 

plates,  145 

plasma,  145 
Blood   corpuscles,   140 

enumeration   of,    141 

source  of,   143 
Blood  flow,  rate  of,  179 
Blood    pressure,   173 
Bleed  vessels,  nervous  control  of 

189 
Body  fat,  source  of,  115 
Brain,  256 
Bread,    105 

Breathing,,  mechanism  of,  209 
Bright's  disease,  232 
Bundle  of  His,  165 
Butter,    106 

Calcium,  120 

Calcium  salts  and  coagulation  of 

blood,  148 
Calorimeter,    85 
Calory,  84 
Capacity  of  lungs,  216 


313 


314 


INDEX. 


Carbohydrates,  24 

food   values   of,   84 

metabolism   of,    106 

relative    metabolic    importance, 
113 
Carbon    dioxide: 

effect  of  oxyhsemoglobin,  202- 
206 

mechanism  of  exchange,  205 

production   of,   197 
Cardiac  cycle,  events  of,  167 
Cardiac  muscle,  163 
Cardiac    depressor   nerve,    187 
Centers,   vascular-nervous,   187 
Cerebellum,    274 
Cereals,    105 
Cerebrum,  268 

function  of,  in  modifying  re- 
flexes, 270 

localization  in,  269 

relation  to  receptor  system,  269 

sensory  areas,   272 
Cheese,  106 

Chemical  composition  of  body,  19 
Chemistry,    of  bile,    73 

of  foods,  104 

of  gastric  juice,  64 

of  pancreatic  juice,  71 

of  saliva,  46 

of    urine,    229 
Childbirth,    307 
Cholesterol,   24 
Chordae  tendineae,  163 
Chyme,  68 
Ciliary  muscle,  283 
Circulation,    180 

diagram  of,  159 

influence  of  arteries,  172;  of 
cocain,  196;  of  gravity,  194; 
of  haemorrhage,  194;  of  ner- 
vous system,  184;  of  nitrous 
oxide,  195;  of  respiratory 
movements,  183 

pulmonary,   182 
renal,   233 

time   of,    179 

venous,  178 
Circulatory  system,   anatomy,  159 
Circumvallate    papillae,    295 
Clothing,   136 

Climate,  effect  of  temperature,  137 
Coagulation  of  blood,  147,  148 
Cocain,  196 
Colloids,  32 
Complemental  air,   216 


Condiments,   107 
Cones   of   retina,   287 
Consciousness,    268 
Consonants,   227 
Contraction  of  muscle,  300 

tetanic  contraction,  301 
Co-ordination,   function   of   cere- 
bellum,  274 
Cord,  spinal,  245 
Cords,   vocal,   225 
Cornea,   282 

Corpora  quadrigemina,   257 
Corpuscles  of  blood,  141 
Corti,  organ  of,  291 
Coughing,   214 
Cranial  nerves,  259 
Creatinin,   230 
Cretinism,  126 
Cream,   106 
Crying,  214 
Crystalloids,  27 
Cystine,  112 

Deglutition,  55 

Dentrite,  241 

Determination  of  blood   pressure, 

174 
Diabetes,  117 
Dialysis,    27 
Diaphragm,  relation  to  breathing, 

210 
Diastole    of   heart,    167 
Diastolic  blood  pressure,  174 
Dietetics,  99 
Diet,   suitability  of,   102 

fundamentals   of,    103 
Digestion: 

bacterial-intestine,   76 

of  cellulose,   76 

necessity  of,  37 

in  intestine,   71-75 

in  mouth,   39 

in  stomach,  60 

object  of,  37 

resume    of    digestive    ferments, 
82 
Direct  pyramidal  tract,  248 
Disaccharides,  24 
Ductless  glands,   124 
Dyspnea,   221 

Efferent  nerve  paths,  250 

Eggs,  106 

Electrolytes,   28 

Enamel,  action  of  saliva  on,  51 

Energy  balance    (see  Metabolism) 


INDEX. 


315 


Enterokinase,   74 

Enzymes,  34 

Erepsin,    75 

Erythrocytes,  141 

Eustachian   tube,   294 

Excreta,  endogenous  and  exogen- 
ous,  112 

Excretion,  from  lungs,  206 
renal,   229 

Exercise,  muscular,   and  metabol- 
ism,  114 

Exogenous  excreta,  112 

Expiration,  209 

Expired  air,  composition  of,  218 

Eye  {see  Vision) 

Fat,   chemical   composition  of,   24 

food  value  of,  84 

of  body,  source  of,  115 

structure  of,  24 

relative     metabolic    importance 
of,  113 

metabolism  of,  115 
Ferments,   34 
Fertilization,  303 
Fetus,  nutrition  of,  306 
Fever,  137 

Fibrin,  source  of,  147 
Fibrinogen,    147 
Flavor,  307 
Foods,    common    composition    of, 

104 
Fovea  centralis,  288 

Gall  bladder,  71 

stones,  74 
Ganglia,  241 

s'pinal,  242,  277 

sympathetic,  242,  277 
Ganglion,  definition  of,  241 
Gasserian,   261 

semilunar,  191,  278 
Gas,  absorption  of,  by  liquid,  199 

partial  pressure  of,  199 
Gases  of  blood,  201 
Gas  exchanges,  in  lungs,  217 

in  tissues,  198 
Gasserian  ganglion,   261 
Gastric  digestion,   6;> 
Gastric  juice,  constituents  of,  64 
Gastric  secretion,  control  of,  61 
Giantism,  131 
'"'.lands,  ductless,  124 

gastric,  60 

mammary,  238 


pancreatic,   71 

salivary,  39 

sebaceous,  238 

of  skin,  236 

sweat,  236 

thyroid,  125 
Globulin,   23 
Glomerulus,  232 
Glottis,  225 
Gluten,    104 
Glycogen,  116 
Glycoprotein,  23 
Glycosuria,   116 
Goiter,   128 
Graafian  follicle,  304 
Growth,  curve  of,  97 

Hair-eells  of  cochlea,  242 
Hoptophore  group,  150 
Hearing,  292 
Heart,   anatomy   of,   160 

augmentor  nerves  of,  184 

heart  block,  165 

cavities  of,  160 

change  in  form  of,  161 

contractions,  maximal,  163 

influence  of  salts  on,  166 

inhibitory  center  of,  187 

inhibitory  nerves  of,  185 

nerves  of,   184 

passage  of  beat  over,  164 

pace-maker  of,  164 

physiological     peculiarities     of. 
163 

position  of,  160 

refractory  period   of,   160 

rhythmic  action  of,  163 

sounds  of,   169 

valves  of,  162 

vascular  mechanism   of,   166 

work    of,    172 
Heart  valves,  162 
Heat,  animal,  sources  of,  134 

value  of  foodstuffs,  85 
Hematin,   141 
Hemorrhage,  194 
Hemoglobin,   141 

absorption  of  oxygen  by,  201 

chemical  nature  of,  141 

estimation   of,   141 

influence  of  acid,  202 
of  carbon  dioxide,  202 
Hiccough,  214 
Hippuric    acid,    112 
Hormones,   38,   124 


316 


INDEX. 


Hunger,  81 
Hydrogen  ions,   30 

measurement  of,  31 
Hydrogen  electrode,  31 
Hydrochloric  acid  in  gastric  juice, 

64 
Hyperglycsemia,   116 
Hypothyroidism,    128 
Hyperthyroidism,   128 

Immunity,  Ehrlich's  theory  of,  150 

specific  nature  of,  151 
Immunization,    151 
Impregnation,  305 
Infection-resisting  mechanism,  150 
Inflammation,  149 
Inhibitory  nerves  of  heart,  185 
Inorganic  salts,  metabolism,  119 
Inspiration,    209 
Internal  capsule,  248 
Internal  secretion,   125 
Intestinal  digestion,  75 
Intestinal  juice,  75 
Intestine,  large,  movements  of,  79 
Intestine,  small,  movements  of,  78 
Ions,  28 
Ionization,   28 
Iron,  120 

Katabolic  processes,  84 

Kephalin,   148 

Kidney,  blood  flow  through,  233 
blood  supply  of,  233 
minute  structure  of,  232 
nerve  of,  233 

Knee  jerk,  251 

Lactation,   238 

Lacteals,  155 

Lecithin,  24 

Lens,   crystalline,   283 

Leucocytes,  movements  of,  144 

function  of,  145 
Lipase,  in  gastric  juice,  67 

in  pancreatic  juice,  74 
Lipoids,  23 
Liver,  excretory  function  of,  70 

glycogenetic  function  of,  117 
Localizing  power  of  retina,  289 
Locomotor  ataxia,   254 
Lungs,  changes  of  blood  in,  217 

movements  of,  213 
Lymph,  movements  of,  157 

formation  of,  156 

glands,  158 


relation  of,  to  blood,  155 

resorption  of,  157 

vessels,   157 
Lymphagogues,  156 
Lymphocytes,    144 
Lymph  nodes,  158 

Maintenance  food,  99 
Malpighian  capsule,  232 
Malpighian    pyramids    of    kidney, 

232 
Mammary   gland,   238 
Mastication,  53 

saliva    and,    54 
Material  balance  of  body,  91 
Measurement  of  arterial  pressure, 

175 
Meat,   106 

extract,   107 
Menstruation,  304 
Mental  process,  273 
Metabolism,  general,  83 

influence  of  atmosphere,  87 

muscular  work,  87 

surface  area,  87 

basal  heat  production,  87 

specific  dynamic  action,  87 
Metabolism,  special,  108 

carbohydrates,  116 

fats,   115 

inorganic  salts,  119 

proteins,  108 
Middle  ear,  292 
Milk,  composition  of,  105 
Micturition,  236. 
Monosaccharides,   24 
Motor  area  of  cortex,  269 
Mountain  sickness,  222 
Mouth,   digestion  in,   47 
Muscles,   300 
Muscle  sense,  275 
Muscular  elasticity,  301 
Muscular  energy,  source  of,  199 
Muscular  tone,  253 
Muscular    work,     expenditure     of 

energy,  99 
Myopia,  286 
Myxoedema,    127 

Nausea,  58 
Nerve: 

abdueens,  261 

auditory,    264 

cranial,  259 

depressor,   187 


INDEX. 


317 


facial,  263 

glossopharyngeal,    264 

hypoglossal,   266 

inferior  maxillary,  261 

oculomotor,  260 

olfactory,  298 

phrenic,  219 

sciatic,    219 

spinal  accessory,  265 

trigeminal,  261 

vagus,  265 
Nerve  impulse,  239 
Nerve  paths,  afferent,  246 

efferent,  250 

method  of  tracing,  245 
Nerve  plexus,  240 
Nerve  system,  239 

sympathetic,   277 
Neurones,  intermediary,  247 
Nitrogen  equilibrium,   94 

balance  sheet,  94 
Nucleoprotein,   22 
Nutrition   (see  Metabolism) 
Nutrition  of  embryo,  306 
Nutritive  value  of  foods,  104 

Obesity,  treatment  of,  95 
Oculomotor  nerve,   260 
Opsonins,  153 
Optical  defects,  285 
Optic  thalami,   247 
Organ  of  Corti,  291 
Osmosis,   28 
Osmotic  pressure,  28 
Oviduct,    304 
Ovulation,  305 
Ovum,  304 
Oxidase,    198 
Oxidation,  in  tissues,  198 

as  source  of  animal  heat,  199 
Oxygen,   absorption  of,  by  blood, 

201 
Oxyhsemoglobin,     effect     of     COn 
on,  205 

Pain,   245 
Pancreatic  juice,  71 

composition  of,   74 
Pancreatic  secretin,  72 
Pancreatic    secretion,    control    of, 

71 
Paralysis,   255 
Parathyroids,  125 
Pepsin,  64 
Pepsinogen,  64 


Peptone,  21 
Peristalsis,  79 
Perspiration,   237 
Phagocytosis,  152 
Physico-chemical  laws,  26 
Physiological  division  of  labor,  18 
Physiological   properties,    18 
Physiological  systems,  19 
Pituitary  body,  131 
Platelets,  or  plaques,  of  blood,  145 
Plasma,  blood,  145 
Pneumogastric  nerves,  265 
Polypeptide,  22 
Pons  Varolii,  246 
Postsphygmic  period,  167 
Potassium    sulphocyanide    in    sa- 
liva,  47 
Precipitins,  151 
Pregnancy,  306 
Presbyopia,  286 
Pressure,   arterial,    175 
intrathoracic,  211 
osmotic,  28 
Presphygmic  period,   167 
Properties  of  body,  physical  and 

physiological,  20 
Proteins,     chemical     composition 
of,   21 
compound,  22 
insoluble,   23 
irreducible   minimum,   96 
nutritive  value  of,  94 
relative    metabolic    importance 

of,  113 
requirement  of  body  for,  100 
simple,   22 
sparers  of,  95 
subdivisions    of 
Proteose,  21 

Protoplasm,  composition  of,  19 
primary  constituents  of,  19 
secondary   constituents   of,    20 
Ptyalin,  47 
Puberty,   304 

Pulmonary  circulation,   182 
Pulse,  use  of,  in  diagnosis,  180 
tracings,  180 
wave,   121 
Purin  bodies,  110 
Pyloric   sphincter,   control   of,    68 
Pylorus,  67 
Pyramidal  tracts,  248 

Range  of  voice,  226 
Rate  of  blood  flow,  179 


318 


INDEX. 


Reaction   of  blood,    32 

of  body  fluids,  30 
Reason,  faculty  of,  273 
Reciprocal   inhibition,    254 
Receptors,   151,   244 
Red  blood  corpuscles,  141 
Reflex  animal,  characteristics  of, 

compared  with  normal,  251 
Reflex  arcs,   240 
Reflex    action,    252 
Reflex  paths,  244 
Reflex  time,  250 

Reflexes,  function  of  spinal  cord 
in,  250 

types  of,  250 
Renal   secretion,   232 
Reproduction,    sexual,    303 
Reproductory    organs,    accessory: 

female,  304 

male,  305 
Residual   air,   216 
Respiration,   197 

artificial,    214 

control   of,    221 

external,  207 

influence  of,  on  circulation,  213 

internal,  197 

nerves  of,  219 

volume  of  air  in,  225 
Respiratory   center,   219 

exchange,  204 

movements,  211 

organs,  207 

quotient,  91,  216 

reflex,    219 

sounds,   213 
Rickets,   120 
Rolando,  fissure  of,  269 
Roots  of  spinal  nerves,   246 

Saliva,  character  of,  46 

dental  caries  and,  51 

function  of,  44 

neutralizing  power  of,  49 

reaction   of,   48 

tartar  formation  and,  51 
Salivary  calculi,   52 
Salivary  glands,   39 

nerve  supply  of,  40 

nervous  control  of,  42 

secretion   of,   39 
Scratch  reflex,  251 
Salt  hunger,  120 
Sea  sickness,  277 
Sebaceous   glands,   258 


Secretin,  gastric,  63 

pancreatic,  72 
Secretion: 

control  of,  38 

gastric,  control  of,  61 

milk,    238 

pancreatic,    control   of,    71 

salivary,  control  of,  42 

sebaceous,   238 
Secretory  process: 

hormone  control  of,  38 

nervous  control  of,  38 
Semicircular    canals,    bony,    275 
Semilunar  ganglion,  191 
Semilunar  valves,  160,  162 
Semipermeable  membrane,  27 
Sensory  areas  of  cortex,  272 
Shivering,    138 
Shock,  193 
Sight,    279 

Skin,  function  of,  236 
Smell,  297 
Sneezing,  214 
Solutions,  isotonic,  30 

hypertonic,  30 

hypotonic,   30 
Sound,  loudness  of,  226 
Sounds  of  heart,  169 
Special  senses,  279 
Specific  dynamic  action  of  foods, 
87 

Tartar,  52 

Taste,  296 

Taste-buds,   296 

Tectorial  membrane,   292 

Teeth  and  fifth  nerve,  262 

Temperature  of  body,  134 

Temperature,    effect    of,    on    mus- 
cular  contraction,  135 

Temperature  sensation  zero,  245 

Temperature  sense,  245 

Temperature,     bodily,     regulation 
of,  135 

Tetany,   128 

Thorax,   contents  of,   207 

movements    of,    in    respiration, 
211 

Thrombin,  148 

Thrombogen,    148 

Thymus,  133 

Thyroid  gland,  125 

Tidal   air,   215 

Touch,  sensations  of,  244 

Toxins,  bacterial,   149 


INDEX. 


319 


Toxophores,   150 
Trigeminal  nerve,  261 
Trypsin,  74 
Trypsinogen,    74 

Urea,  108,   230 
Uric  acid,  110,  230 
Urinary  organs,  232 
Urinary  salts,  nitrogen,  108 
Urine,  ammonia,  108 

excretion    of,    229 

nature  of  excretory  process,  233 

Vagus  nerve,  action  of,  on  heart, 

183 
Valves  of  heart,  162,  170 
Varicose  veins,  179  ^ 

Vasoconstrictor  nerves,  190 
Vasodilator  center,  187 
Vasodilator  nerves,  191 
Vasomotor  tone,  194 
Veins,  blood  in,  178 
Velocity   of   blood,    177 


Ventilation,    223 
Vision,  279 

color,   290 

stereoscopic,  290 
Visual  defects,  284 

treatment  of,   285 
Vital  capacity,  216 
Vitamines,    121 
Vocal  cords,  false,   224 ' 

relation  of,  to  pitch,  225 
Voice>  224 
Vomiting.  58 
Vowels,   227 

Water,  proportion  of,  in  body,  20 
physiological  properties  of,  20 
Wheat  flour,    104 
White   blood-corpuscles,    144 

Xanthin   bodies,    110 

Yawning,  214 


COLUMBIA   UNIVERSITY 

as  provided  by  the  rule,  Ztu,u        ''='*<=  °f  borrowing, 
range„.e„t  wiL  tlL'uLria'nlnXe."  "'  '''''''  - 


C2e(63e)M50 


QP34 


Pearce 


P312 
1916 


